Why can't I level up?

[Danielnator]

I can't seem to get the skull rank. Still on the double crossbones...

I had a friend who has worse stats than me on EVERYTHING, and a worse K/D ratio... And he got the skull.

[InfestLithium]

Make sure that your K/D ratio is meeting a certain criteria. The Skull emblem is relatively easy to get, but it may be out of your reach if you play matches with a high down count. Play lots of consistent games in a map where you're confident in, try new strategies, and have fun doing them! After a few matches (or a few days) of playing, you hit the Skulls in no time.

Also, you can use this guide as a reference:

viewtopic.php?f=100&t=26010