I would start by looking at the section expert for that section and see if the Keep Together option, or New Page Before option is set. Uncheck it if any of them are selected.
Check the section before it, if applicaable, and see if the New Page After section is set. Uncheck it if it is selected.
If the problem still persists, open up the subreport, then open up the section expert there.
See if the Keep Together option is set anywhere and uncheck any of them that are selected.
~Brian