I did a Passage Guide on 2 Samuel 21:19 and it didn't include the JFB commentary (a.k.a. A commentary, critical and explanatory, on the Old and New Testaments) in the commentary list.
I'm assuming it's because the commentary comments on sections instead of verse-by-verse. In this case, the relevant section in the commentary is 2Sa 21:12–22. The reference box only says 2 Samuel 21:12 while in this section.
I'm not sure if it's the PG or the resource that needs fixing, but something does.
If I run the PG for 2 Samuel 21:12, JFB is displayed.
BTW, the short name for JFB in Logos 4 is the oddly chosen "ONT".