Most of the other Pokemon I have with me are above 20. Would it have anything to do with the higher lvl Pokemon on my team? I'm playing Fire Red and I have the Boulderbadge.
If it won't listen to you then you obviously traded it from another game and you don't have the badge required for it to obey you. Either catch a Caterpie on your game, or get another gym badge so that it will obey you.