I purchased an "EX" M2 a couple of weeks ago from KEH (my first M). It's been perfectly smooth, until yesterday, when I noticed that the shutter button has started to get a little sticky. I'll wind on a frame, then let the camera rest or walk around with it a bit, and when I go to press the shutter, it'll be blocked as if I haven't advanced the film. When I move the film advance lever out to its "ready" position, this frees up the button. Time for a CLA, maybe?