I forgot to mention that I never just put Dodger's food bowl down. He works for every piece of kibble.
Try some self-control exercises with Tucker. Put him in a sit-stay, then put a treat on the floor. You don't have to be in heel position. He doesn't get the treat until he makes eye contact with you. At first release him right away. As he improves, only release after a few seconds of eye contact. Ideally, you want to get to the point where he never breaks eye contact, even as you toss the treat on the floor.
I'd also try to mix things up a bit and make training fun. Try to learn a new skill or trick every week - you'd be amazed how positively humans approach trick training compared to obedience training.
I think you are doing everything right...from the sounds of it, I think it is just a maturity issue!!