Check to make sure that the filter pad isn't loaded with schmutz. It can cause enough of the water to flow through the bypass and not through the normal channel where the wheel sits.
Check to see if there are small removable inserts that the wheels axle rest in. If so, pop them out and clean them. If no inserts, clean the slot(s) were the axle sits. Clean the axles too.
Check your impeller also. make sure that it's not all gummed up. Clean if necessary.
Make sure that your tank water level is topped up too.