Developing ideas

A couple of weeks back, Rachel asked this:

I was wondering how you work with and extend story ideas without getting bored? Because I have a habit of writing or imagining “moments” that really interest me, certain people or situations that last a page or two, but when I try to extend the idea to a full story I almost immediately become bored with it.

When a would-be writer has this problem, there are a couple of possible reasons, and what you do about it depends on which reason fits your parti...