I'm no neurologist, so this is just a guess. I've noticed very much the same thing.
If I stay in exactly the same position, I can often get back to the same dream, or
at least a very similar one. But if I move, definately not. My thoughts are that
maybe when you tilt/move the position of your head, blood flows from the recently
active area of your brain to a new area. And I do know that different parts of the
brain store different genres of ideas (i.e. the biggest one being right brain v. left).
It makes sense to me, albeit it is a very basic explanation. Whattaya think?