I'm having the same problem. It says 100%, but its actually not. And i wish the fires acted like fires on ships. I still dont get why they dont. Is the big guns supposed to add fires on all enemy tiles like the musketeer spell, or it might not? Sometimes it doesnt add anyway.
Try fighting some random low lvl enemies to find out where on the board the fires can appear and where they cannot. (Half of the board is your's, half of it is the enemies i'm assuming)