They often get jammed up with old lubricant. It is NOT simple to get at this part, and unless you have a fair bit of camera repair experience and lots of confidence, I would not recommend you attempt the repair, but here's what you need to do.
First, remove the top plate, and the back plate of the shutter crate. Remove the self-timer lever, the leather from the left-side front panel, and the cover plate over the self-timer mechanism.
Next you need to remove the top casting, which I seem to recall is held on by five or six screws. Then CAREFULLY remove the shutter assembly held in place by two screws on top and one at the bottom. Only then can you get at the self-timer mechanism which is held by two screws, top and bottom. Running it through an ultrasonic cleaner is usually the best, but just soaking in solvent and carefully working the wind up and down will usually suffice. A bit of light oil on the shaft ends is a good idea at this point.
Assembly is the reverse of the above of course, and the only thing that is likely to be tricky is getting the shutter assembly back it. This part likes to hang up on the circular baffle around the focusing helical. Remember where all the tiny little shims came from, and DON'T FORCE ANYTHING!
Best of luck,
Dez