The first time, though of course he doesn't know it's the first time, everything went exactly according to plan. Crowley got the baby, right on time. He took it to the nunnery, and didn't stop to talk to the man waiting outside. He observed the whole thing, and made sure no funny business happened. At the end of the night, the Antichrist went home with the Dowlings.
It takes 600 more tries to get it going exactly according to the right plan.
Aumerle lives to regret many things.
Of all the prisons in which she's spent time, Lotus Pier may be the kindest.