Is it a 90 degree corner? To do that turn moveobjects off. (I'm not sure if that matters but it might)
First put down the counter on one side of space where a different counter will change into a corner. Then you put one on the other side. You put the corner piece in last. The most common problem is that the original counters are too far apart, so just change one of them a bit and try again.
This is only for 90 degree corners though.