I have a 1963 Sears cruiser, three speed, which I bought very recently. On my third ride, the middle speed stopped working, and went loose; there wasn't any "traction" when I pedaled. It happened as I rode in a particularly hilly part of town. I'm new to biking, so perhaps I shifted gears at the wrong time? The highest and lowest gears still work fine, only the middle speed doesn't work. Before buying this bike, the mechanic realized that the lowest gear had this same problem, and fixed it pretty quickly. Do I simply need to tighten or loosen the changing mechanism? Or do I need to replace it?