We had a similar issue, when I first got the camper I would take my finger off the slide button as soon as I heard the motors load up and I knew it was closed.
Eventually the motors got out of sync and the slide would bind.
I did the old RTFM and found out on my particular setup, you hold down the button until the controller shuts the motors off. To resync them I had to close the slide, open it 6" or so, close it again and hold the button until the motors shut off...did this 2 or 3 times and haven't had a problem since.
There are a few different slide systems though...maybe these guys know which one you have. But that is how mine works.
I know some of them DON'T want you to hold the button down once it's closed.