Try the stuff called Forbid (they have other names for it like Yuck, etc). Put it into the food of the dog (it's like a powder). Pet stores, your vet, online, or catalog all carry it. It's gross but some dogs do it. Basically the powder makes it taste gross... you would think that it already tastes gross but...
It will tell you how much to use per weight of the dog.
There can be many reasons for why, but this usually will stop it. I have heard of a few dog (very few) that it didn't work but this is a good 1st step.
Good luck and remember what we think is gross isn't always gross to our dog. Just like we wouldn't lick our butt and then turn around and eat our food either.