I've done all four special jobs from Delvin. From what I've read, I should be able to speak to Brynjolf and he will make me guild master. When I speak to him he replies with something along the lines of he's busy right now.

Is there something I'm missing aside from completing the special jobs?

Did you complete the Thieves Guild questline and gotten Nightingale armor. If not, do that. If you did, I've heard of a glitch where you can't be guildmaster if you did the jobs after the main thieves guild questline.

That must be it. I have so many quests going on right now I can't really keep track of them. I'll have to check it out when I get home tonight. Thanks!

This is a repost:
Yeah, this happened to me this morning on the thieves guild quest line, having played til 6am. I'll try to explain everything that I did up to the bug. First of all, I completed the Devlin and Vex side quests first. Then I started the thieve guild main quest line. I only got up to "Dampening Spirits" before Maven will not initiate the next quest to the Honey brewery. I incorrectly wrote Bethesda tech support. They apologized, but said I would indeed have to go back to a save file, which I'm sure I have, but still... what a drag. That's anywhere from 4 - 6 hours spent from probably 2 - 6 in the morning, something that I'd like to hold on to. I should also include, Louis Letrush was continually attacking me through out the game every time I set foot in Riften. Also, I ended up assaulting Maven a hand full of times before knowing that she would be a quest giver. And then I was kicked out of the guild twice because of npcs attacking Maven, apparently at my behest according to the guards of Riften -another bug, but not game breaking. Lastly, I failed at least the first two quests it takes to become a part of the guild (couldn't find the right ring to steal off the lizard and plant on the elf, or something to that effect), but was still accepted into the guild.
So I hope this never happens to any of you. Happy gaming.

So, it looks like it doesn't matter if you do the side quests or main quest line first for the thief guild, bugs either way. Also, make sure you have done 5 jobs per city, not including Riften, you don't have to do any there. So 20 jobs total and then the side jobs are done.