Something to add. I've been playing with a different family on the same save and a few of them also had 12 or more traits by the time I was done with them without triggering it. Then, last night, I moved that family in with my original sim through manage households. I went in to play and as soon as the house loaded with the combined family, the achievement triggered. So I have no idea what is going on there, but I have the achievement now??? I don't know if moving the original out and back in would have done it because until that point I hadn't moved any of them after having 12 traits (but in the first save where it worked I had never moved her).
Having this same issue, and I was wondering if DLC traits could be the problem?
