Sorry I worded it wrong.. When I say "mission", I was meaning "battle". I have completed battles 1 through 10, all missions within each of them. I think the issue is I'm running the original Tie Fighter game, and not the Collector's Edition.. Battles 11, 12, and 13 (?) are not on the original, I don't think. I'm going to try to snag a copy of the Collectors and copy my .plt over to see if that will work.
I'll let you guys know, thanks for the quick response.