That is very common is old bows, when the glue or epoxy wasn't as good as today's, the best thing is to work the handle off totally and reglue using the new types of epoxy and clamping it for 48 hours.

I have succesfully finished unglueing some like that using lacquer thinner and acetone, but don't get it on the laminates ,just apply it to the handle on the open area of the handle.
Make sure the wood is dry from the lacquer thinner before you reglued it.