I think it was easy to teach Kaige bow as he does it naturally. He likes to play a lot in the bow position and sometimes he'll even just sit there with his butt in the air.
I trained him by putting a treat in my hand & have my hand in front of him but almost to the floor to lure his front half down. I'd say "bow" & he'd start to lay down... luckily once he heard "bow" instead of "down" he would get a little startled and pause for a minute. That worked out good cause when he paused it was the perfect time to reward him.

Then I started to work on "catch" before he knew bow 100% and now his bows are a little weird. lol I have also noticed that he doesn't do bow on his own as much as he used to.

Frequent but short training sessions are best. 5-10 minutes at a time a few times a day. And if you work with Jamie when Charlies not around they do concentrate more on you.