Newbie question: my dog switches to another behaviour!?

Hi, hoping this question is not *too* silly. I've got a little dog who enjoys clickertraining and has a few tricks mastered. But often times when I go to try and teach him something new he seems to get it for a while and then switch back to something he knows. I know when he lies down he's usually frustrated, but what about when he starts throwing other behaviour at me? Is he trying to tell me that he's bored with the new lesson or that I need to up my treats for the 'new' behaviour? He has a bit of a delicate stomach so we don't use too many varieties of treats. I'm not sure if I should switch gears when he gives up, or try to keep him going? He lasts about 5 minutes with something new before he goes back to something he's done before.