I had the same thing happen. The cable that attaches to the release lever can come loose. so even though the lever feels like it is working, it will not release the back catch. The fix is not very easy, you must remove the trim piece from the lever area, it takes a very short bit and some way to turn it to remove the screws that hold the trim in place. When the trim is off you can reach in, and manually pull the cable. Once the seat is folded, it is much easier to replace the trim. If you need pictures, I will try to take some tomorrow, and forward them to you.
Greg