The dog may not understand what you are wanting him to do. If he gets up and and is able to reward himself with the treat, then this is what he'll do. So the first thing is to not let him have the treat when he doesn't do what's asked. Now you have to make it clearer what the desired action is. Luring him into a roll is one option, as if using your hands to gently flip him over, whilst saying "Roll". Also, if he regularly rolls naturally to itch himself, use a cue word such as "Roll" when he does this, then reward him. Eventually he will connect his action to the cue word, and better understand what you want.