Good luck, been there. Best dog I have owned did the same thing (not on bumpers though) at about the same age. Bramble went like a bullet and then looked at the frozen bird as if to say now what? Tried rope with mixed results. I was devastated.
Great advice so far! could be teeth, doubt it at that age though. I tired live birds too, and though the pup was fired up, he still dropped them while trying to "real him in"
Ultimately, FF made a huge difference--
Good luck--welcome to the forum. I am a rookie trainer. Lot's of good advice here.