In order to restore the Thieves' Guild to its former glory and assume the title of Guildmaster, you must perform 4 'special jobs' across four major Holds of Skyrim: Whiterun, Markarth, Windhelm, and Solitude. In order to get a special job, you must first complete 5 smaller jobs in that hold. To get a job, talk to Delvin Mallory or Vex. The small job targets are randomized across the major holds and a tally of the small jobs is not visible to the player. Jobs can be accepted and quit at will. After completing 5 jobs in a particular Hold, Delvin Mallory will offer a Special Job in order to increase the Thieves' Guild influence in that Hold. Once you have completed all four Special Jobs, you can talk to Brynjolf to officially assume the title of Guildmaster. Completing a Special Job also increases the Gold available to all fences up to a maximum of 4000. Quitting a special job given by Vex or Delvin carries a penalty of a random gemstone being removed from the player's inventory.

I was given a special job from Delven. Then I grabbed a job from Vex as I always leave the hideout with two jobs. When I got outside Delvin's job was missing and I could only do Vex's job.

Is this a bug? It has happened more than once but this is the first time a special job disappeared. So does this ruin the whole restore the guild quests? I can't seem to get a special job again.