What is your ISO setting. Make sure you are using a low setting and try again. My guess is your ISO is too high for the full power of the flash in manual. Also try dialing the flash power down to 1/16. I have a 5DmkI. I have never used in P mode but do use Av mode quite often. If my ISO setting is too high and the flash is in manual the shutter speed will go to 200 which is the max flash sync speed and I end up with over exposure.