I think something needs to be said that is often overlooked, possibly even feared and so it is rarely uttered.

What in your story is making you lose interest?

Think back to when you were last really engaged with the narrative and work forward. I guarantee you, if there is a part of the story you are losing interest in... you will be losing your reader as well. Likely not on the same scale, but it will happen. While your disinterest might span a few weeks and theirs only moments... a moment is all it takes to break immersion and remind them that they are in fact, not running for their life. They are on a busy, smelly train and it's late and they're hungry and now your story is just some person writing about things. You can't afford to lose the reader.

Often times stories die in the valleys, the quiet middle bits where you are diligently building sub plots, fleshing out characters, and all the million and one things you must be conscious of.

Watch the Coen brother's movie Burn After Reading, or at least a few scenes. They build tension in those middle bits simply with music. High thriller tension music all the time and for things like getting a paper. It's nonsensical but you don't even notice it at first and you wonder what the hell the big deal is about getting a paper. Then you realize that the coen brothers are keeping you on the edge in the extremely mundane parts so that their major events can be played any way at all and it will be a tension release. Stick and the carrot.

If part of your story is boring or tedious to write... it will be boring and tedious to read. Now that's not saying everything needs to be high drama and action. Instead use that time to weave subplots. Raise questions and suspicions. Mess with the playbook. Call and audible and see where it goes. Which characters know what and what is their role in all this. Be hitchcockian. Keep people in the dark, but show them the footprint. It's good practice in writing by the seat of your pants, you don't even know what this character is doing but you know it can't be good.

Steven Pressfield is very good at this. The Hot Gates was a perfect example of a narrative that started and simply didn't let you take your eyes off the page.

Even if dropping it for a while is just 'your process' try to analyze if there was something you could do to not let the pressure up (there is ALWAYS pressure.) Think of lateral ways you could have jumped into say backstory, or a particularly revealing scene about character that no one is expecting. Bam now you have deep motive running underneath and not just a sub plot. I'm pretty sure i've read easily upwards of 6-7,000 books. There are some from when I was young that I still remember vividly, entire scenes even. They varied from stories about hunting dogs to kid detectives and secret Nazi missions on D-Day. They all have one thing in common (I went back and checked)

They stay in motion.

Just like a shark will die if it stops moving... So too dies your story, There will of course be exceptions as per human individuality... but don't think of it as something to prove wrong. Think of it as a challenge to get our of your comfort zone. Give yourself a healthy dose of army inspiration.

Must not be very good if even you don't want to read it.

Be tough on yourself, everyone else will be. Readers are often ruthless. They don't owe your book anything. In fact you owe them for the money they paid. This advice was articulated in a much better way I'm sure by a number of my professors over the years. It has helped me immensely and as a result I don't stop writing because i can't stay interested. I have to stop writing/reading because knowing what I know is like being the only one in the room who has seen the Game of Thrones episode coming on. I CAN'T WRITE ANY FASTER AND IT'S KILLING ME. It took some getting used to and some rather frank conversations with myself about what is in the story... and what should be in the story... but it works, even as just an exercise to get warmed up or to try and keep your current roll going.

Another tool for the toolbox. Good luck and remember...

WRITERS WRITE.