As I said in the post above, Halo uses TrueSkill. This is a rather complicated system that uses some ideas that are good on premise, but I don’t personally like the system as it is apparent to me that it does not allow for players getting better overtime and thus having a higher skill. Read about TrueSkill here: http://research.microsoft.com/mlp/apg/Details.aspx#How_to_Update_Skills

As mentioned in that article, your skill is determined by two things, mean (peak point) - Mu and standard deviation (spread) - Sigma. You can read a bit more simplified summary of these concepts here: http://www.bungie.net/forums/posts.aspx?postID=14414600

Since you’ve played a considerable number of matches (and I am not sure how many you’ve played in Lone Wolves only), I believe that the ranking system has zeroed in on your sigma, thus very tightly limiting the range in which the system will allow your rank to fall. Due to this, it could be some time before your sigma will change, but when it does, it should be easier for you to get to 45, and after that to 50. Please note when reading the two articles that I linked to that the levels (Mu and Sigma) of the opponents you are facing are also very important when calculating whether or not your levels will change. (This is easily observable if a level 1 in team doubles pairs with a friend that is a much higher level, the level 1 will raise in skill very quickly if the team consistently wins)

I hope this helps.