I believe there is a tempaban on those who quit games early and hence can't get into any online matches, nothing to do with the servers that are running fine, I tried this with both Xbox live accounts I have, one I quitted several matches before completion, the other I started from scratch and works fine to prove is nothing to do with the connection, either yours or MW3s, rather a "secretive" way to ban those who annoy others.
EDIT: I think I found the problem, this may occur on XBox Live if you dashboard, they prevent you from connecting again to multiplayer
Trialled two different XBL accounts
in reply to Luke's comment, exactly what happened to me
· 8 years ago