I've completed all the challenges for the Precision Calling Card a few months ago and it never unlocked for me. I just recently started playing this game again and noticed that. What gives?
This only means that you have to do the said achievement again so that it can be unlocked, I actually have no issues with unlocking the said operation. Just need to make sure that every games you're playing are finished.
But how do I know which one? It shows that I've done all of challenges but still tells me I only have 18/21...
Hmm... It seems like this is really a weird. It seems like you have to try them out again and make sure to double check every Precision operations there.
I know that some of the challenges can be a bit tedious to complete, bro. But you'll really need to try it again. The unlocks for CoD games are system generated. Also, if this is the precision master, there are some other things you need to unlock as well. Here's a link I found about this calling card: Tips on Completing the Precision Master & Return to Sender Challenge in Black Ops 2 (HD) - YouTube
Look on the brightside though, atleast you'll just need 3 more.