Try flipping the door lock to lock, then unlock. While holding the key in that position, press the button. You may have to do it several times. The lock mechanism jumps out of track in the latch, and it won't open no matter how much crap you spray in there. You should feel the button almost grab, but then slip. Flip the lock to lock, then unlock. Repeat. Finessed mine for years before I finally replaced the latch.
Replace the latch before you lock the door again. Use a back door latch.