I don't know if there is an issue with some accounts, or if it's the way in which you are doing your grinds, but I have never wanted for a key. I don't think I have ever gone more than 6 strikes in a row on the 320 list without having one drop for me. I was grinding basically all day yesterday for Imago Loops and got 6 keys in a little over 6 hours, 2 of which dropped Loops. If it helps, I am dropping into 320 match making, either using a booster or not, depending on how long I'll be on. I stay in the list until I get a key, and then drop out to go run the strike I'm farming. I never drop to orbit manually, and always just wait for the game to take me to the results screen and drop me back in the queue. It seems like the streak bonus drastically effects the chances of having one drop. Other than that, best of luck, there's no way that I know of to make it go faster than just staying in the playlist till one drops.