Twenty-three night-shift workers at an Amazon delivery facility in Chicago are accusing the company of not paying them for overtime during Prime Week earlier this month.

The employees, part of a recently formed organization of warehouse workers called DCH1 Amazonians United that advocates for better working conditions, filed a complaint with the Illinois Department of Labor on Wednesday and delivered a letter to their site manager stating their concerns.

“We’ve followed all applicable wage and hour laws, and are committed to speaking directly with employees to help them understand their pay. We offer a variety of ways for employees to share their questions and concerns, and our leadership team is directly engaging with employees at the site on this topic,” said Kelly Cheeseman, a spokesperson for Amazon.

Prime Week is one of Amazon’s biggest and busiest sales weeks — the company sold 175 million items over the course of two days earlier this month. During that time, a record heatwave hit Chicago that made these workers’ Amazon facility, DCH1, unbearable to work in. The workers behind the complaint say that management dismissed them six hours early on July 19 and promised to pay them for their full shifts.

They initially celebrated when management agreed to let them leave the facility over six hours early with pay, as BuzzFeed News first reported. In a video posted online, workers are heard cheering when a site manager made the announcement. But that joy has turned to anger.

When these workers checked their time cards, Amazon had counted those six hours in the following week’s pay period — meaning workers would not be paid overtime for those hours. Since workers officially complained to management, several say they’ve seen adjustments to their timecards to shift the hours to the correct week — but they still aren’t being paid overtime.

In addition, 17 of the 23 workers in the complaint alleged that Amazon incorrectly docked their attendance on the day of work in dispute. In interviews with Recode, these workers said the company counted the six hours of dismissed time as an absence from work.

Several long-term employees (Amazon calls them “blue badge”) say the time was counted as unpaid time off. Temporary employees (“white badge”) told Recode the company classified the time as “absent” and gave them an absentee point in its system. At their warehouse, if blue-badge employees rack up more than 20 hours of unpaid time off in 90 days, or white badges accumulate 15 absentee points, they can be terminated, according to employees who work at the facility.

The workers’ complaints come at a time when Amazon warehouse workers across the world have alleged that they are overworked, underpaid, and mistreated. Before Prime Week, some of the workers who are part of the DCH1 Amazonians United group had set up a petition on Coworker.org that called for air conditioning at their facility, health care for workers, and higher pay during busy sales weeks. And the same week that these Amazon employees in Chicago say that the company failed to pay them for overtime, Amazon fulfillment center workers in Minnesota went on strike on Prime Day to demand the company lower productivity quotes, employ more full-time employees rather than temporary workers, and better address on-the-job injuries.

“It’s not right for a trillion-dollar company not to pay their workers what they’re owed,” Ruby Quintanilla, a sortation associate at DCH1 who is one of the workers who filed the complaint, told Recode.

Quintanilla and two of her coworkers, who also signed the complaint, told Recode that before Prime Week, site managers encouraged warehouse staff to take on extra shifts, totaling up to 60 hours a week. Anything over 40 hours would be considered overtime and paid out at time-and-a-half of a worker’s normal pay rate. (Amazon’s minimum pay rate for employees is $15 an hour, and most sortation associates listed in the complaint make around $15.50 an hour, according to Recode’s sources.)

But now, according to the complaint they filed Wednesday, workers say they’re seeing several overtime hours at the end of Prime Week incorrectly placed in their timesheet system on the following week and counted as regular hours, rather than overtime. They told Recode that the company later moved the hours to the right time period, but still didn’t pay overtime for them.

“The deal it sounds like the company made with workers is that they would be paid for that weeks’ work,” Paul W. Mollica, a lawyer with Outten & Golden LLP who has been practicing employment law for more than 30 years in the Chicago area, told Recode. “And [the employer’s] responsibility is that they pay overtime during the week when it was actually earned. Assuming that these employees are covered, either by Illinois or federal law, they do have a very good claim.”

The group filing the complaint estimates that workers are losing about $50 in pay on average for the one-week pay period in question. For many workers who are living paycheck to paycheck, this can have a significant impact on their finances.

“You work so hard the entire week, thinking, ‘I’m going to get out of whatever debts I can this week’ — that way you don’t have to worry about things for a little while,” said Quintanilla, who estimates she’s owed more than $100 in pay she hasn’t received for her hours during Prime Week. “And it turns out to be the complete opposite. Nothing but headache. And nobody seems to be able to help us.”

Quintanilla said she has spoken to her manager, human resources, and Amazon’s Employee Resources Center (ERC) about the problems, but has yet to see them resolved on her time sheet. A group of about 10 workers from DCH1 Amazonians United, which Quintanilla is a part of, delivered a letter to their Amazon site manager Wednesday night asking for the hours to be corrected. They also asked HR to be present on the warehouse floor during shifts for the next two weeks so that “all of these and other issues can be resolved.”

They feel particularly frustrated because they were encouraged by management to take on extra overtime shifts during Prime Week to fulfill the spike in consumer demand, only to be, as they see it, penalized for helping out.

“I would tell management that we are trying to keep the promises of your customers. Please give us more compassion and respect,” said one employee claiming missing overtime pay, who asked not to be named. “We’re not robots. We are trying to help you and your company just as much as you can help us.”

Recode and Vox have joined forces to uncover and explain how our digital world is changing — and changing us. Subscribe to Recode podcasts to hear Kara Swisher and Peter Kafka lead the tough conversations the technology industry needs today.