The problem is often due to debris such as dust or sand between the release magnet and armature face.
With the bottom cover off, you can easily see the magnet (usually wrapped in black tape) and armature that control the exposures, but that's not the one to worry about -- except for the tiny little spring holding it's armature against the magnet. You do NOT want to accidentally push that little spring down off the armature!
No, the release magnet (usually wrapped in blue tape) and armature are further down inside, on the rear of the shutter's mechanism plate. If you cut a small strip of clean paper (2-3mm wide) and carefully insert it between that magnet and armature, put a small drop of alcohol on the paper so that it wicks down, slowly wind the camera until the armature begins to move showing that it is being moved into contact with the magnet, then carefully draw the paper out, you may succeed in wiping away any debris from their mating surfaces.
Here (I hope!) you can barely see, at the left end of the blue-wrapped magnet, the end of the silver armature.
Here's the back of the shutter. At about 5 o'clock of the lens, you can see the release armature to the right of the blue magnet.
I've used a slip of pink paper to illustrate where you will want to insert the paper for cleaning the faces of the magnet and armature.
Here's the view from the bottom of the camera. Once the paper is in place, put the drop of alcohol on it, wind slowly until you see the gray arm just begins to rise, then slowly pull the paper out.<edit insert> The gray arm will move after only one or two clicks of the brass pawl at the very beginning of the wind -- more than that, and the paper will be pinched, and cannot be removed. If the strip of paper gets pinched, just continue winding until the shutter trips, and stop as soon as you see the gray arm move again. <end edit/insert> Don't tear the paper! If you do, you can either completely disassemble the shutter to remove the torn piece, or simply blow it deeper into the space behind the shutter. Chances are it won't hurt anything (we hope!).
Here you can barely see the tiny spring resting against the back of the exposure armature -- the one you DON'T want to push out of place!
