it's a while since I last did one, but there's a bit of wriggle room in how far you press it in. If you press the body into the sleeve far enough that the key won't fit, you press the body out a wee bit until it does, and vice versa. Looks like the body on yours was pressed in too far. Just put it in a press, supporting the sleeve with the jaws 1/2 way out and carefully press on the top of the body (not the jaws) a smidge, should be a piece of cake. Like I said, I've never done a ball bearing chuck, but they're still pretty simple. Take your time and if possible use an arbor press.