bones won't rig

I’m having an annoying and bizarre problem.

I was able to weight paint a few bones of an armature to a mesh but now whenever I try to weight paint, it won’t “stick” so if I leave the bone and come back, it no longer works. I’ve also noticed that it’s stopped making vertex groups for these new bones. Could I have possible have hit some kind of swtich for the mesh that doesn’t allow any more bones to be rigged onto the mesh?