You might want to try another back, to isolate whether the problem is in the body or the back. If you can find anyone near you with an S2 from which to borrow a back, perfect. Unfortunately we are about 35 years past the point when you could walk into a local camera shop and buy an S2 back off the shelf! So if you can't borrow one, it would require some patience to find one on eBay, and sometimes the prices there are silly. But it might help you diagnose things. And once you fall in love with (and repair) this camera, you'll want a second back anyway. (Either to carry one color and one B&W, or to carry two different speeds of B&W.) I am still looking for a reasonably priced second back for mine...
--Dave