Finally figured out why my dishwasher wasn't working right

Same symptom on my KitchenAid a couple years back. It would fill, the heater would kick in, and then just sit there humming with water pooling at the bottom. I assumed the worst and almost ordered a whole pump assembly, but before spending the money I pulled the bottom rack and the filter, reached into the sump, and found a shard of a broken mason jar lid wedged under the wash impeller. It was jammed just enough that the motor spun freely but the impeller never caught. Pried it out with a flathead screwdriver and a pair of needle nose pliers, checked that the impeller turned by hand, and the machine went back to normal that same night. Total cost was nothing. So before buying parts, I'd feel around the sump for hard debris, especally if anyone in the house has broken a dish recently and you kept running it. Also check that the spray arm nut at the bottom is snug. On mine it had worked slightly loose, which let the arm wobble instead of spinning. Hand tight plus a quarter turn was all it needed.

Might not be your issue, but it took me ten minutes to check and saved me a big repair bill.

I had almost the same thing happen on our Whirlpool last winter, but the fix turned out to be nothing to do with the pump or the valve. The tub was only filling about halfway up the heating element before the fill shut off, and the wash motor woud just spin without grabbing any water. It turned out the float in the corner of the tub was sticking from years of detergent gunk, so the machine thought it had enough water when it clearly didn't. I popped the float out, scrubbed the shaft with an old toothbrush and vinegar, and it's filled properly ever since. Might be worth checking your water level before you spend anything. If the water is sitting well below the bottom of the spray arms when the wash starts, the pump has nothing to push no matter how healthy it is. Mine would hum and even sound like it was working, but the arms never budged because they were sitting in air, not water.

Also, the mason jar lid thing in the earlier reply is real. We lost a chopstick down there once and it made the same silence. The sump area catches all kinds of stuff you would never expect.

Reading this makes me side-eye my own machine a bit. When mine runs I hear the water fill up, then a little click, then nothing for a while before it finally starts sloshing. I always figured that quiet stretch was normal and it was just warming the water first. Is there supposed to be a pause between the heating part and the spraying part, or should it start moving pretty much right away?

To answer the fella asking about the pause, yep that's normal. These machines fill, then run the heater a stretch before the wash motor kicks on. Back in the day you could hear the timer dial clicking through each step and you'd learn the rhythm of your own machine like a song. Mine still sits quiet for a few minutes after the click before it starts sloshing. Long as it eventually gets going and the water's hot when you crack the door mid-cycle, you're fine. If it never gets going at all, that's when you start pulling things apart.

I agree with the idea about debris catching the impeller. I had a dishwasher struggle with exactly that issue, and it wasn't a big chunk of glass like you described. Instead, it was just a small, hardened lump of mineral scale that had built up around the wash rotor. That sediment got caught in the vanes and prevented the motor from spinning with enough force to circulate the water properly. Getting that scale out of the sump was the only way to get the circulation back, so checking for buildup is defintely a thing to consider.