Doesn't? start the under new management quest, what do i do? i have been doing research and i havent found an effective fix... does anyone knows how to fix it. i have just finished all 4 missions on the thiefs guild and more that vex and the other guy gives me. but when i talk to Brynjolf he always says that he is busy....

When a quest breaks due to a bug, I start a new quest.

I have heard of this happening, but don't know of a definite fix. "Under New Management" is supposed to automatically start (come up in your journal) after you have finished the quest "Darkness Returns" and completed the last of the four "city influence" quests" * that Delvin gives you.

*The four city influence quests: "Imitation Amnesty" (Whiterun), "Summerset Shadows" (Windhelm), "Silver Lining" (Markarth)", and "The Dainty Sload" (Solitude).

Also, you may have done this already, but if not, try the suggestions that may apply to your situation under the "Bugs" heading on the following web pages:

http://www.uesp.net/wiki/Skyrim:Under_New_Management#Bugs

http://elderscrolls.wikia.com/wiki/Under_New_Management#Bugs