I just noticed this today, but it must have been this way for some time. I've had Believer's Church Bible Commentary in my Library for several years, and it has been prioritized (as a whole set) in my Library for almost as long as I've had Logos 4. Today I noticed that the label for it in my prioritization list says "0 volumes" even though I have all 19 volumes of it and they're prioritized as a series (title is boldfaced in Prioritization pane):

And indeed it doesn't come up in my passage guides. I can fix it by deleting this entry and re-prioritizing it, but a bug exists somewhere -- how did it get into this situation in the first place? Unfortunately I don't have logs of everything I've been doing since I prioritized that series. And perhaps today's log won't show enough to let you track down the problem, but I felt obligated to report it anyway in case you can figure out this puzzle.
Log file: 6136.Logos4.log