I would not recommend bamboo for a maple bow. I would also not recommend a bamboo backing on any fourth attempt in bow making. Gluing up a bamboo backing requires a lot of skills; only adds more risk factors. A split stave of maple is excellent material unbacked. Remove the bark, and there is your back.
In what form has the stave been in for the past four months? It is most likely not yet dry enough. You can get it to floor tiller (so it just start bending). But it needs further drying for a few weeks before you can start the actual tillering.
