I did everything I needed to do. I did three of Delvin's "Special Jobs" and I'm not not the guild master! Why can't I get this last mission? I did all five missions in the cities I was supposed to do them in. But I have 3 out of the 4 spaces full of merchants.

Did you complete the Thieves Guild main quest?

Yep.

Something is off in that questline somewhere.

There's one guy that my quest log told me to talk to, but whenever I go to talk to him he tells me he's busy, maybe later. I go back later, he's busy - forgot his name, he's the main guy that helped you chasing down the previous' GM killer.

I've completed all the other quests and I'm the guildmaster.

> Three

Yea its four, you're missing one.

Enthir, from the College of Winterhold?

Did you by chance start the CoW questline but not finish it? There's a point in that questline after which CoW NPCs will no longer help you with anything else or discuss other business with you because there is now some major **** going down in the College and they will be preoccupied with that until you help them deal with it (by finishing their questline).

He is talking about Brynjolf and yea as I've said you need to do FOUR quests, you are missing one.