Yeah, that would be a good bet. :P

I found out what I did wrong. I played on Casual. Not even Regular. So I'm going back and playing on the hardest difficulty.

That's weird, when you complete the storyline, you should receive all storyline related achievements + the difficulty achievement you started on.
Are you sure you fully completed the game? aka you're in mercenary mode now.

I have a question for Just Cause 2. I completed Mission 7 on Regular, no achievement. Do I have to complete it 100%?