I will try it however the quest I am trying to get to start is called the Dainty Sload and I have console quest ID memorized in trying get this quest to go through what I have repeatedly tried was SetStage TGTQ20 10 or even TGTQ20 20
The thread is marked with the topic of the quest Under New Management (i was a bit confused with the title and the topic) the command above is for that but if you want the quest The Dainty Sload the id is TGTQ02.
In the other comment you had TGTQ20 not TGTQ02 but to start The Dainty Sload it should be -
setstage TGTQ02 10
After looking at a few threads in other places with Google some people seem to say it doesn't work for some unknown reason, the only kind of solution if thats the case seems to be jumping to Under New Management when you've done all the other quests needed for it.
As for why, the Thieves Guild quest line is extremely buggy i'm afraid it is listed on the bugs section that the quest just won't be given, just like the Civil War its like they tried to make things to complicated for the game to keep track of sometimes.