The bassinet DS sleeps in is elevated at the head. I also noticed that he wiggles down. I think it's from him kicking his feet in his sleep. We will be transitioning him into his cradle soon (once DH finishes it) and then he will be lying flat. Hopefully he still sleeps now that he's used to a slight incline!
DS has been in his crib since 2.5 weeks and he sleeps on his belly (I'm aware of the risks...). I couldn't ever put him on an incline. He kicks so much that he wiggles all the way up his crib. Little mover! He's 9 weeks old now.
Yes we elevate our crib- but only slightly. She moves a little but not much. Try less of an incline. We use a folded beach towel so it is very easy to control how elevated it is.
Re: Crib mattress at an angle