While the other solutions do a good job of explaining how to gain XP and level up, the information on how to overcome the glitches are buried in comments. I'll start off with my theory on how the glitch occurs in the first place, which seems to be due to when you reach rank 14.
I had to hit rank 14 a total of 3 times. The first 2 instances were in results screens, when my double XP and ribbons were being calculated to my total. The 3rd time was in-game, which is why I think it actually worked that time. So if you're getting close to rank 14, try to make sure you get it after earning XP in a match as opposed to afterwards.
Right, now onto actually fixing this. The old solution was to delete the multiplayer update and play on unpatched servers, which have since been patched and integrated into the rest of the official EA servers. Should this glitch on you now, the only option is to reset all of your multiplayer stats by deleting your EA Origin account and trying again.
WARNING: DOING THIS WILL RESET YOUR STATS ON EVERY SINGLE EA GAME. THIS MEANS IF YOU ARE IN THE MIDDLE OF BOOSTING/PLAYING A MEDAL OF HONOR, BATTLEFIELD, TITANFALL, STAR WARS, FIFA, NEED FOR SPEED OR ANY OTHER EA GAME THAT USES ORIGIN YOU WILL LOSE ALL MULTIPLAYER STATS!!!Right, with that out of the way, the process of deleting Origin. You have to go to
https://help.ea.com and choose issues until you can speak to a representative. Tell them you want to delete Origin, give them whatever reason you want. They'll tell you everything I just told you in the warning and inform you that account deletion is permanent. Once you give them the all-clear, get back on Warfighter. You'll be back to rank 1 with nothing unlocked, so grind up to 14 again for another opportunity. As I stated initially, I had to delete my Origin account twice before this buggy little pile finally unlocked for me.