Sidetracking
It's easy to come up with some random topic to write about. And it's also easy to start writing about it. But sometimes halfway through you either run out of ideas or notice you have diverted writing about something completely different.
Without a good plan and a topic that has enough subject matter, it might be hard to keep on the subject even for a few hundred words. An idea might feel like easy to write about, but does it carry all the way to the end.