The wheels internals are shot! my starting to do that after a few times it was caught on wires and struggle for the whole battery to get out, finally the transmission cassette stripped and the robot now can revers out of the dock but as soon as it moves forward the stripped wheel stays put and the good wheel turns the unit in circles until battery dies.
Need to replace the broken wheel hub. To test if this is the case for you, go into manual mode and see if you robot can move in a straight line by giving the forward moving command.