Whirlpool WDT710PAYM3.

The very top (little) arm spins and the bottom (big) arm spin, but the middle arm (under the top rack) does not spin (leaving dishes dirty).

I replaced the main circulation pump - no change.

I checked the float switch - working properly.

I checked/cleaned all the arms - no obstructions, seem fine.

I checked the inlet water screen - clean, no obstructions.

I rigged a switch so that I could trick the unit into thinking the door was closed and I could run it while watching (with the door open). The bottom arm spins, but the middle arm doesn't spin at all (but water does come out the holes in it).

The water fill level was below the heater element, so I tried adding about a gallon of additional water. Now, the middle arm does spin, but spins very slowly (is that normal?).

I have sufficient water pressure to the solenoid, but it seems to just dribble into the tub. Is it possible I have a solenoid that isn't opening far enough, leaving me with insufficient fill? Isn't the purpose of the float valve to make sure it doesn't stop filling until it gets enough water into the tub?

Any thoughts on next steps?

The float switch is just a safety to prevent an overflow.

The fill is just a timed event.

It does sound like the fill valve is bad.

Update: The new valve (finally) arrived. I was initially worried, as my "try to blow through the valve while it's energized to assess flow rate"-test didn't show any difference (compared to the one I had assumed was bad). But when I actually hooked it up to water, approximately 2x the flow rate was observed. This has, in turn, fixed the problem with the upper spray arm not rotating.