K12 Door Window fault

Anyoen dealt with a door window that will not shut?
The motor appears to be OK, perhaps the runners are worn out, or the regulator.
It rocks from right to left, and I have to pull it towards the rear, when closing to get it to close correctly - otherwise there's a gap at the rear of the window
My mechanic messed around with it, but couldn't get it to work correctly. Tried silicone spray, which helps but doesn't fix the problem.
Any ideas?