Halo: Reach level up!?

Ok so I am rank Sergeant Grade 2, and I am having a very hard time leveling up and and I earn like NO credit every game (we are doing firefight and gruntpocalypse), my friend is a private rank 2 and he gets like 4,000Cr every game, but i get only 50. I am wondering why this is. If you can help I would be very thankful!