While I don't know the "whys", my BH does this same thing occasionally. With mine, the little bar that comes up when the hammer is back, gets hung up. If I tilt the muzzle upwards it come back like it's supposed to. I don't know if there's a spring behind it or what, but it does it so INFREQUENTLY that I don't pay any attention to it anymore. Oh, the only times I had it happen has been while it was unloaded. Don't know why that has anything to do with it, but thought I'd throw it out there.
HWD