But even when guests are nudged to leave tips for the housekeepers, it doesn’t always work.

In 2014, two longtime housekeepers at the JW Marriott Santa Monica Le Merigot recalled, guests were regularly leaving cash tips when they checked out of their rooms, a result of the hotel chain taking part in “The Envelope Please,” an initiative started by the nonprofit group A Woman’s Nation to make it easier for customers to show appreciation to housekeepers. Envelopes were placed in 160,000 Marriott-managed hotel rooms in the United States and Canada meant to be filled with notes and tips for cleaners.

Within a few weeks, though, the envelopes vanished. “We heard that some guests felt the hotel was demanding tips for us,” Blanca Guerrero, a housekeeper at the Santa Monica Marriott, said through a translator.

Tipping norms, or the lack of them, may be especially unfair to housekeepers, who arguably do more for guests than park their cars or push the cart containing their dinners. Angela Lemus, a housekeeper at the Wyndham Boston Beacon Hill who makes $19.91 per hour, said through a translator that in addition to scrubbing tubs and taking out trash, she sometimes has to clean blood or other medical waste from rooms. Ms. Guerrero and another housekeeper at the Santa Monica Marriott, Aurelia Gonzalez, who make $15.66 an hour, said their responsibilities include cleaning not just the inside of rooms but also the balconies attached to them.

Most housekeeping workers are women, a disproportionate number of them minorities and immigrants, according to Unite Here!, a labor union that represents thousands of hotel housekeepers in North America. Ms. Gonzalez was born in Mexico, and Ms. Guerrero in El Salvador. Though their wages are above the median hourly wage of $11.37 for hotel housekeepers reported by the Bureau of Labor Statistics in 2016, the profession’s earnings are less than the pay for housekeepers in other industries, like hospitals ($12.74 per the Bureau of Labor Statistics).

Pay rates vary widely by region. Wage Watch, a company that tracks wage and salary information for the lodging and gambling industries, found that a housekeeper in a New York City hotel can expect to make an average of $29.41 an hour, while one in Charlotte, N.C., may earn an average of $10 an hour.