One of the biggest reasons an audience isn't laughing (or why a reader stops reading) is that they have unanswered questions.
Let's say you mention something in passing, especially something really loaded like a previous marriage, a recent breakup, or the time you spent in prison.
You may have moved on to another topic, but the audience/reader is still thinking about the door you cracked open but didn't walk through. What were you in prison for? How many marriages have you had? Why did you break up? Are you heartbroken?
Mentioning something that emotionally-loaded is like putting a gun onstage. The audience keeps wondering if and when it's going to go off.
That's why we developed the 'back mic' at the Un-Cabaret. Because a lot of the material was being done spontaneously or for the first time, the writer-performers didn't necessarily have time to think things through. That's why we always put a second microphone in the back of the room that Beth can use to talk to the other comedians while they're onstage and actually ask the questions that the audience might have in the moment. Here's a great example from one of Patton Oswalt's performances.
I can tell you from producing hundreds of Un-Cabaret shows and editing hundreds of hours of these performances that those questions often prompted great, hilarious, spontaneous storytelling. And a lot of that material then became part of the performer's 'act'.
Beth is able to do that for people in a workshop format at the Un-Cab Lab and I can do it for people in my personal coaching, but you can also sometimes find a friend or peer to do it for you when you 'run material by them'. And sometimes you can do it for yourself by really paying attention to what you say onstage and listening to how the audience reacts.
This information is also contained in You But Funnier, an mp3 download to help you deal with unanswered questions and other reasons the audience isn't laughing.