Well, to get to the point, the Tough As Nails achievement is refusing to unlock for me.

I played through the story on normal to get used to the game, went and played multiplayer, and decided to start hunting achievements soon after.
I got every achievement, apart from Tough As Nails. Time to play through the story on hard.

I played through it once, in two sittings (one for each side), and the achievement didn't unlock. I thought that I might've somehow missed a level.
I went back, created a new character (as by now, my previous character was level 20), and started to play through the story again, in hopes of grabbing myself the final achievement for Brink.

I've just completed the entire story mode in one session, from lunchtime earlier today, 'til now. I didn't do the levels back to back, as I had to stop for drinks, food, etc, but I finally got there- but the achievement hasn't unlocked.

Anyone else having this problem, or know what the problem is?
Do I need to play all 12 missions back to back? or do you actually have to complete the What-Ifs, even though the Achievement states otherwise?

Cheers in advance.

have done the security and resistance campaigns on hard with the same character?

Yeah, on the same character, I've done them twice now, I think. First time round, I wasn't sure if I missed a mission, so I created a new character to play through them all again, and made a list to tick them off when I completed them, just to make sure I didn't miss one out again, and I definitely did each mission (excluding what-ifs), but still saw no achievement pop. I've only tried doing them on Hard solo, I haven't played them through on Online Versus story mode.

Also, I think I might've put this in the wrong section of the forum, I only noticed last night after posting that there was an xbox specific section.