Dark floors or dark carpets absorb light such that the machine may regard them as cliff which it should avoid; thus, these areas may be neglected If there is dust on the cliff sensor, the cliff sensor may be wrongly triggered for non-dark floors. Please use soft & dry cloth to carefully clean the 4 cliff sensors on the bottom of the machine and then try again Some places may be missed if the wall sensor is dirty. Please clean the wall sensor in the right of the machine.