Pop 'em apart and look at the springs. You might have one one with a grommet instead of a spring. If that's the case, the easiest thing to do will be to trade one stick to somebody for one that matches your other stick. You might find that one stick is missing it's pivot cone or is assembled wrong. In that case you'll have to replace the missing part or re-assemble correctly. Or, you might just have a broken, incorrect, or worn-out spring, in which case a pair of new, matching springs would be your best bet.
re: Handles- look on Ebay for Wico handles. You'll find balltops galore. For replacement bat-tops, Happ Perfect 360 handles might well fit. I don't have a set of those handles, so I've never tried it, but the P360 is the successor to the stick you've got, and many of the parts are interchangeable. Might be a source for springs, too.