Rear wiper; intermittant parking

When giving the rear screen a wash via the switch it wipes approx 4 times but at times it parks in the middle or any where on the screen.
Again when I switch the rear wipe on ie to clear, not wash,again it parks half way through a wipe almost anywhere on the screen.
I have to use the switch off then on to get a full wipe and to park it at the bottom of the screen.
If i leave the switch on it will start a wipe
but intermittantly park discriminately on the screen.
The motor appears to be working. But what could be the problem ??