I had a similar issue with the initial upgrade. The card level requirement and class level requirements were met, but the upgrade was locked. The trick is... your class level requirement needs to be ONE LEVEL HIGHER than what it says. So.. for the first upgrade... it says you need class level 10... but in reality...the upgrade only UNLOCKS when you reach level 11. Thus, in your case, if the class level requirement says 15... when you turn level 16 the unlock will occur. Hope this helps!
Thus... put simply: Your CLASS LEVEL needs to be one LEVEL HIGHER than what it says on any requirement BEFORE the upgrade actually unlocks... even though it LOOKS like you should be able to upgrade now!
