Well there is a notorious bug that is in morrowind that continued onto the expansion titled Morroblivion. If you get the code book for the fighters guild before starting the thieves guild then you can not ever start the thieves guild quest line. This is fixed if you put in this code! :)!!!
Setstage fbmwTGAdvancement 1
Took awhile but figured it out
Have fun !! :D
This was never a bug to begin
This was never a bug to begin with, but whatever suits you.