That's not ineffective, it looks fine to me, just a little bit dark. I think it might be that your shutter speed was a little fast, but I am not certain, and especially since you think you were using the P mode anyway...
Many people seem to have this 'teething problem' with their Canon flashes, but being a Nikon guy (and not having experienced such problems with my NIKON flash
), I can't offer a hugely detailed answer...
Maybe have a read of this:
Flash