Are you using a training table?I've started FF with my 8 month old black lab. I'm working on hold right now. He really like to gnaw on the paint roller. I've been told just to tap the bottom of his chin when he does this. It'll work for a few seconds but then right back to the gnawing. This is my first time doing FF, will this go away the longer he's working on it?
I would go back to having the dog "hold" the first two fingers of a gloved hand.
For a gnawer a paint roller may not be an appropriate next step, perhaps a hard plastic bumper may be more suitable
Look this over it may give you some ideas