Yea, I am not surprised this failed. You need to nail this, all of it, not just where it is popping off the wall, everywhere. 18 gauge brad nails, use a paintable wood filler to fill the holes, sand, then paint, you will not see any nail holes.

Pin nailers work well, but don't buy a pinner just for this.

For future reference, in certain applications, just gluing things works well. This is not one of them, you still need to mechanical fastening.