I'm not sure if it makes any difference, but how are you triggering the YN-560 II to fire off camera?
Does this happen on every shot, or is it OK at full or 1/2 power on the FIRST shot, but dark thereafter?
If this is the problem (IE OK on first, but dark therafter), it may be you are not giving the flash enough time to recharge - the YN-560 II is pretty fast, but still needs 2-3s after a full power flash before it can fire again.
What shutter speed are you using? (too high a shutter speed can cause problems with flash).