Are you sure it isn't clogged? Are you getting good, hard suction from the hose? If not, your suction line is plugged. If so, read on.
I have a Hayward Pool Vac XL, which at least looks almost the same as the Navi 50, and probably works on the same basic principle. There have been occasions where I've taken off that bottom cleaning plate to access the turbine and haven't found anything. And then what I did was dismantle it -- removing the four screws that hold the base to the rest of the unit. After removing the base, I've found things. Leaves stuck in crevices that weren't visible from the inspection port. Acorns jammed up in the works where they weren't seen. Twigs that got caught up in the innerds. I've found acorns to be the most insidious.
So if you haven't removed the bottom assembly from your cleaner yet, give that a try. If it's still not working, you may have gotten some small pieces of debris that have gotten inside the clockwork. I've never tried opening mine up that far, so I can't give you any specific advice in that respect. But even if the clockwork got jammed up, as long as the turbine spins freely and the platforms with the feet rock back and forth, the vac should move.
