I've been having the same problems recently.

Fortunately, if I use miles from a *A program that still permits changes after departure, i just book the outbound as i want (from a gateway, but I always either fly ex- sfo or lax) and then book the return sometime in the future.

then once i am at my destination, i simply change the return about 4 or 5 days out. I only ever ask for LH over UA by choice on transatlantics. this has worked and supports the above theories about the time frames LH realeases seats, but it won't work using UA miles however.....