Is the button pushing in at all.
1. If it does not push in at all, then there is either something wrong with the lock (like maybe it is locked

) or there is something foreign jammed in the way.
2. If it pushes in with little or no resistance then the armature of the button may be missing the latch.
3. If it pushes and seems to have the proper resistance, then it is more likely to be a problem with the latch.
If it is scenario 1 try Zedman240's suggestions.
If it is scenario 2 also try Zedman240's suggestions.
If it is scenario 3 try reverendzed's. But before you do that try a bit of exra force while holding the button in.
Good luck