How to make proper crimp joint
Yellows should hold perfectly on #12. 

Could be the conductor is not exactly #12, I've seen it happen, especially with foreign wire. IE: not made in North America.
Could also be that your crimper is out of adjustment, if it has that as you suggest.

Are you crimping in the correct area?

I will say, sometimes you can use blue terminals on #12 just fine and they do hold really well.