Many of the scripts I’m reading at the competition level are very unsophisticated in the plotting of their B stories. Many don’t have B stories at all. Your A story may be well plotted and the characters may be clearly and succinctly drawn, however, if there aren’t interesting B stories dense with subtext that inform and dovetail with the A story, then just one A story doesn’t feel like enough to support a feature.
Another way of looking at the B story is that the non-protagonist characters should all still have their own worlds that coexist with the world of the protagonist. Their stories run simultaneously with that of the protagonist. If you only have stuff happening with the protagonist, the world won’t feel as full.
I read quite a few romantic stories wherein the romance is the A story and there is no B story. That for sure is not enough to sustain a feature.
The A story should focus on the protagonist accomplishing a physical, externalized goal, such as winning the race, surviving, escaping, outrunning the cops, getting the job, traveling around the world, getting out of a country safely, etc.
The B stories can inform about the secondary characters or the love interest in a way that dovetails with the A story. Usually it is something shifting in the B story that then allows the protagonist to accomplish his goal.
The best way to really learn about how to construct a B story for any given genre is to watch movies within your genre, beat them out, and then study the specific structure. Then you’ll have a working template for how another writer constructed similar elements.