I seem to be able to write plot summaries straight from head to page but I then seem unable to go any further and write the actual story/novella/novel, for example the one I wrote for 2009 NaNoWriMo. This one just fell out of my head fully formed with these words:

[Maybe set in the future, maybe an AU (alternate universe).]
It is known [here and now] that weather and many other external influences from the environment affect health, minute by minute. That these can have subtle yet wide ranging and[/or] disastrous effects [be that] physical, mental, psychological or physiological.
There are tried and tested [proven], practical, chemical and mystical measures to both combat and control these effects and 'the people' manage to pursue their lives with many fewer intrusions of this type.
So, when a new illness/disease/disorder/malaise begins to appear amongst one sector of the population that cannot be explained by the usual reasoning or treated by established methods, the environmental, medical, mystical and government specialists are at a loss to know how to proceed.
When a stranger arrives, dressed differently and with a strange look in her eyes, the confusion and panic only escalates then they hear what she has to say.

Now, if only I could follow through and write the stories in my head…