I rode with a friend of mine yesterday and asked her to take a look at my attempts at a shoulder-in with Pete. She'd seen me do them during lessons so I was hoping she'd be able to tell me what's so different that's making it not work.
She told me I should try it at the trot.
If you can headdesk in a saddle, I did it.
Oh, and at the trot? It worked beautifully.