Bifold closet doors not closing properly

We had bifold closet doors hung in a bedroom and they do not close properly. They both pop open in the center of each hinged door - and will not stay closed flat. They will sit for a while - then pop open again. Very annoying. The door opening is 5'- with two 28 1/2" panels (14 1/4" each). Is there some sort of gadget that will hold them flat???