There have been other threads of people saying more drop ships aren't spawning in since the patch in early december. Just make sure you have killed all the minions, which means you can't be hiding in the 'cheese' spot in the shrieker room doorway. And you did release phogoth, right? I'm sure others will post phogoth needs to be almost dead, but I don't think that's really necessary. I've done this twice, and the time I did it solo, phogoth had over half/about two-thirds his health when Xyor showed up.