I just took mine out for the first time last night, as I installed new cargo mats that go under them. It took some doing, but after shifting them around I was able to get them back installed and working. Try pulling out on the handle, or actuating the lever next to the handle. I forget what exactly did it, but I worked with it for a couple minutes and got it.
One of mine doesn't lock in the folded-up position. One locks, one just falls back down.
This is for a suburban, but I think the seats are the same.