My garage door sometimes acts funny. When closing the door, it stops
after about a foot and retracts back to the top. Opening the door is
fine. It doesn't always do this, but it's becoming annoying.

I have straightened out the rails as much as possible to prevent
snagging. I have greased the rails and the chain. I am starting to
think there is some sort of tension problem but I don't know that much
about these devices. Any ideas.