The short version
The producer relationship runs on information, and the designer controls the supply.
What is really going on
Producers cannot protect a schedule they cannot see into.
Scope changes must be logged at the moment they occur to be recoverable commercially.
Dependencies you are waiting on are the producer's job to unblock, but only if they know.
How to handle it
Send a short weekly status: done, in progress, blocked, at risk.
Log every out-of-scope request the day it appears.
Give estimates in ranges with the assumption stated, not single optimistic numbers.
The takeaway
Keep the producer informed enough that they never have to ask twice.
