Make sure you make the cut deep enough. I read somewhere that said that if it doesn't look like a scene from 'Pulp Fiction' you didn't cut deep enough. Cut deeper.
I cut until blood is really flowing....really flowing. if you just see some blood that isn't deep enough, it has to flow.
Then I walk away for a few minutes. The first few times I did it I cut their head off. It took me a few times to settle down and just cut the veins.
Oh, make sure you place the knife under the feathers right under their ear; knife to skin, not knife to feather. If you cut feathers it dulls the knife and you don't get a good clean cut.
good luck.