Well, for me it is the set up. Since I write from my own life experiences, even though it is written in short story form.
Most of them are of a funny nature, or even the philosophy orientated hubs are mostly true life also. This is a style that takes a lot of set up in order to come out right. All the conversations have to be remembered, the places and the things done there need to be right.
The hardest part is keeping the story flowing, without over lapping into another story that hasn't been written yet. This is my biggest problem, with so many different episodes, and the same character's. Picking the story out from the background is the task at hand.