Several things to check. Need to verify what model you have.
There is a screen filter close to the wall connection. Confirm that is not clogged.
Then confirm you have good water pressure to the unit. When you take the net bag off and booster pump on - do you see water jets shooting up towards the opening where the net bag was?
Are the rubber on your wheels gooey? If so, time to replace them.
For the most part, if unit is not moving, it is about getting pressure to the turbine that drives the wheels. Any lose or reduction in pressure will slow the cleaner.