OK, so I bought a used Polaris 9550. Instead of getting a new robot cleaner, I think was taken to the cleaners. The rear wheels seem to move fine, but the front wheels and front brushes spin erratically and when the cleaner gets to a wall, they don't move at all; the rear wheels keep spinning, but it is stuck. After a while, the unit reverses, pivots and moves on (until another wall). The front wheels are what I would describe as "wobbley." I can easily see in between the wheels and the main unit to see the two white gears that should be turning the wheels, but they are so loose that the gears only only occasionally mesh, hence the erratic movement. I watched the Polaris video on how to replace the wheels by pushing down with a hex key in the hole on the main unit and prying the wheel off with a screw driver, but when I push down with the hex key, I don't feel anything give or release, and the wheel definitely does not pry off fairly easily like in the video, so I did not force it for fear of breaking the unit before I can complain to the seller. In what 2 or three paths it ran it picked up a surprising amount of stuff, so I'd really like to get it running and keep it so I can get rid of my old Polaris 180 . Anyone have any insight as to what's wrong with the front wheels?