I had this happen to the release pin on the lens side of a 1.4x teleconverter. Since those have a lever on the side that's attached to that pin I could push it out, but every time I mounted a lens on it the pin would stay in and not lock the lens. At the time I had a friend who was a retired watch maker. Really. As mentioned above, he used a toothpick to transfer the smallest amount of oil to the pin, worked it in by moving the pin up and down, and it worked fine from then on.
