1960 Valiant door latch/no release

Hope someone can help with this. Driver side door will not open with outside handle. Have taken off interior trim to diagnose but am a bit confused. Locking mechanism seems to be working, both with interior handle and key from the outside, but outer pushbutton will not release door. Inside handle does. Latch unit is a bit hard to see, using a mirror to view. Window rail in the way. What possibly could have happened? No slamming shut or other unusual event. Did something break or come off? Where to look? Thanks to any guidance.