Look at the inside of the front wheels. Make sure the small gear is engaging. I had one wheel with worn off teeth, so I changed it with a rear wheel and now both turn. Also make sure water is shooting out of the rear jets, that will tell you if there's enough pressure to it.
I just bought an Aquabot 4WD to replace mine. Doesn't work much differently, but saves energy and cleans easily. Does seem to get more of the walls, and also scrubs.