Help - trigger ass'y stuck on Knight Revolution!

[CedarPost]

I am a relatively inexperienced muzzleloader shooter. I got my old Knight Revolution .50 out to sight it in for this weekend's anterless season in VT. I haven't shot it since deer season of 2020. But before I could get a shot off, the trigger assembly got stuck open! The assembly is pretty simple, and I can't for the life of me figure out what happened, and neither could the very helpful guy down at my local gun shop.

I've found the owner's manual and read it, but didn't find any help there. The latch (labeled "B" on the diagram on page 24) will not open. Advice is very welcome. Thank you.

 

[deermanok]

What would happen if you removed that screw? Would the trigger assembly drop out?
Have you tried spraying some oil or WD40 in the there?
Maybe there's just some crud in the assembly that's binding it.