You can't, as Details A fires before Details B.
Can you reverse them?
The alternative is to insert yet another section, place the subreport in Details A, return a shared variable for use by Details B to use in a suppression formula, and then use a suppression formula in Details C to suppress it if there aren't any rows, otherwise fire it again...
You may be better served to describe the tables, environment, example data and expected output as there may be a better way to handle this.
-k