Several of us have mentioned that Roli's drive wheels sometimes slip, particularly when turning on carpet or other difficult terrain. I found an easy solution.
I was shopping in the electrical department of Home Depot today and came across 3M 3/4" Friction Tape. It is a cloth tape designed to keep electrical cables from slipping around.
I took off Roli's treads, wound two pieces of tape around each wheel and put the tread back on. No more slipping.
You could probably also use the tape they sell at sports equipment stores for wrapping baseball bats and hockey sticks, but I have found that to be very tacky on the back side and may pick up dirt and lose friction over time.