I have no idea why this happens? Maybe there are a lack of players in specific game modes at that given time. Or on that specific server?
P.S I'm writing this after I have just completed the ODST Deja vu achievement. And my was it frustrating. We had 3 players so we needed one more. Easy right? You would be dead wrong. The first person who joined had a bad internet connection so lagged the whole game out. The second was Mexican, who couldn't join as he didn't have a hard drive. (like how does one game without a hard drive?) The third said to me he had the game. He loaded it up only to tell me had had sold it the month prior. So In the end I made a new xbox account, had to pay for xbox gold, because it seemed to use my 1 month free xbox code without giving me the gold. So overall it took about 2 hours finding someone who ended up being myself on another controller. And about 30 minutes playing the game. Oh and i made numerous frantic posts on xbox clubs. Funny thing was when people like a comment asking for help you assume they want you to pick them. Nope. Seems like people like anything. They didn't even own the game for pity sake.
Thank you for reading my little story.