So if i understand correctly, with the "improved critical" feat, a rapier (which has critrange 18-20) should create a crit threat with rolls 15-20 since it "doubles the threatrange" (which should hence go from 3 to 6).
But it doesnt. Even 16 or 17 rolls won't create critical threat rolls.But: Sometimes they seem to do so. Mostly they dont. I am confused.

Also: Can someone explain me what the conditions for flanking an opponent are? Sometimes all my team is hitting one enemy and it wont create flanking... what could be the cause?
I know that the tabletop rules (where you actuall have to "flank" someone) are different from the rules applied in this game. It actually sais in a loading screen: An enemy creature is considered as flanked if it as being attacked by two characters.
But... sometimes it wont work, why? Are there "unflankable" creatures?