I watch an ass-ton of movies, in theaters even, I go every week; I've never heard of "black bag" until seeing this thread today.
Sounds like your "Star-driven

movie" could've used a tad more exposure mr Soderbergh.
That's interesting, there was a trailer for it at every film I saw on the run up to the release. I'm not sure how the arrangements are made for which cinemas or films show particular trailers, but there's obviously not enough time for every screening to have every trailer so there must be some sort of selection process and obviously the film isn't big enough to get promoted at every mainstream cinema in the way that a Tom Cruise movie, for example, might.
I saw a few trailers for "marching powder" which I imagine won't have been promoted much if at all outside the UK. It looks to not be aimed at me at all, or to have much in common with the films I was seeing, so there might be multiple criteria at work - region, film certificate, for sure but also presumably genre, size of release, etc.
I wonder if in addition to Black Bag having a terrible name that doesn't really grab the attention, if the lack of star interviews was a problem (or something that the film didn't have budget for). I'd say Blanchett and Fassbender are easy bookings for chat shows but I don't think I saw them being interviewed recently. It also seems like a really difficult film to promote:
Here's Naomi Harris talking about the film, not being able to say anything without spoiling it, completely failing to sell it and then a clip that doesn't inspire confidence at all.
It's a shame, because I normally complain that I don't want to know what's going to happen in a film, but here I don't think there's enough for people to actually feel compelled to go see it.