<p>i'd whack it a bit on the body near the button with a pencil, or something of similar weight that wouldn't mar the surface of the camera. I bet it's just wedged either due to being a little bent, or by old crusty lube that's in there.<br>
+1 to the suggestion of a CLA. I shoot mine as a hobbiest, sometimes very actively, sometimes not, and recently got them back after a CLA. the last CLA was 8 or 9 years ago. I haven't had any problems with my cameras, backs or lenses over the 15 years or so i've owned them.</p>