Here's the most effective method you will find for curing a dog from "freezing" ("sticking").

When the dog comes to heel, reach down and gently grab the flap of skin just in front of the hip, to the side of the belly. As you give your "DROP" command, just pull up on the flap of skin -- the dog will spit the bird right out. 100% guaranteed

Trying to force the dog to drop will only make the problem worse........an ecollar (in this situation) won't work either. I would definitely advise you to FF your dog in the future, but in the meantime, use the method I described above.