Horsehair on a violin bow can become worn and stretched out with use. It goes without saying that when using your bow it should never be over-tightened, and when finished using it, always loosened. Over time your bow hair will continue to stretch ever so slightly and eventually get to the point where you will not be able to tighten the bow. Time for a new bow or for a rehair. Do also check the screw on the frog which may be worn – but this repair is a job for a profession luthier.