Some plugins don’t make the most of the new hierarchical categories in WordPress (well… new since 2.1 anyways). Most plugins that fashion their own SQL queries take only a single level of a category hierarchy into account.
For example: my blog Spontaneous Derivation, has this partial category hierarchy underneath Fantasy and SF category:
Fantasy and SF [id 1] +-- Awards [id 2] +-- News [id 3] +-- Reviews [id 4]
Most plugins, when asked to work on category 1, will neglect to include the posts under ids 2, 3, and 4; yet all posts in the child categories implicitly belong to the parent category 1.
Here’s how to add hierarchical category support to these plugins, under the cut.
Spontaneous ∂erivation 