I've been searching pretty hard for an answer to this specific question, but if I totally missed it, I apologize.
Some back story:
While I have ASM'd, called boards, and generally run a stage before, I am in the midst of calling my first show completely solo (I have no ASM or other crew). There is also no production manager. The show is small, only 5 actors, and has very few cues.
Since the very first rehearsal, I had little to no contact with the designers, but not for lack of trying. I would have to ask 3 or 4 times via rehearsal reports, calls, and texts before I could get a response to the simplest of questions. Sometimes, I wouldn't even get an answer at all, so I'd have to go to the producer and ask her to contact them. When I expressed my frustration with this system, she explained to me that because I didn't have a relationship with the designers the way she did, they were refusing to answer to me. She also mentioned that it wasn't a reflection on any poor job I was doing, just that they preferred to answer to her. Of course, not coming in contact with me directly has lead to some problems (like when a 10lbs speaker fell off a shelf and onto an actresses' face).
I've worked with several professional companies and I've never had to go through the chain of command quite like this. Am I wrong to think that this is unusual, or does it really just differ from company to company?