One front wheel on my Legend Platinum cleaner frequently stops turning, especially when it hits a step. The other front wheel continues to turn but alone it often cannot climb the step. The drive gear on both wheels turns fine. I've replaced the front wheels thinking that perhaps the plastic teeth had worn down, but that didn't help. The drive gears for both wheels wiggle a little, but the one that doesn't work is no looser than the one that does. Any ideas for a fix that I could do myself?