Agree with JNW that preserve birds are a good substitute when, for whatever reason, you're unable to shoot many wild birds over a dog. That may help with retrieving issues.
With dogs that don't want to drop a bird, it sometimes works to lift them up off the ground by their hindquarters. Will often cause them to drop.
My oldest shorthair (now 13, hunting career ending this season) has had hundreds of wild birds shot over her. Good retriever, does not have a hard mouth, but sometimes doesn't want to drop the bird. I'll use a low level electronic reminder with her, to reinforce the drop command. Works with her, but you have to know your dog's personality. Excessive pressure can result in a dog that will no longer retrieve. At which point about your only option is force breaking, preferably done by a good pro trainer.
