Found this.
Sometimes Brynjolf will get stuck in the Market along with the other market merchants and be unable to leave to meet you at the Ragged Flagon, acting as if he is still creating a distraction, even though you have completed the previous quest. A way to get around this is by pushing Brynjolf to the door to The Ratway (by either sprinting at him or using the Unrelenting Force shout, then yielding/healing hands). Tell any followers you have to wait in Riften, save, activate the door and quickly talk to Brynjolf. (This does not work 100% of the time, so if Brynjolf doesn't appear in The Ratway, load your save.) Once you're in The Ratway with Brynjolf, attack him and run to The Ragged Flagon. Once inside, have him chase you all the way up to the bar, then yield. Talk to Brynjolf quickly, before he can leave to go back to his stall, and he will give you the next part of the quest. Alternatively on the PC you can go to the Ragged Flagon and enter these console codes:
- prid 00020545
- disable
- resurrect 1
- enable
- moveto player