lately what happens to me the most, is in swat. I won more than 20 in a row to go from diamond 4 to diamond 5 and lost the last one and I went down halfway through diamond 4, while my friend from diamond 3 reached onyx 1620
I could be wrong, but I believe how much your csr increases or decreases each game depends on whether you win or lose, as well as how well you did. So if you’re both playing well, but your friend is dropping high kill low death games regularly, they could increase in rank faster than you.
I have no idea why there would be that much of a difference in rank increase, or why you drop so much after losing one game. I’m sure someone else can explain that part though.
If you read the matchmaking FAQ, or ask Josh in the matchmaking forum, you can get a better answer but this is probably because your friend has a higher MMR than you. You play like a diamond 4 so the system will keep you in diamond 4 until you perform like a diamond 5 or better. Just playing more games or “grinding” won’t increase your rank if your not actually improving.