Quite a lot of my reports have been threads where one user has posted multiple times (usually five or less minutes apart) - at a guess, the admin/mods have looked at the sheer numbers reported and decided not to bother.
Hm, that is weird, that user was clearly breaking forum rules and that admins/mods decided not to do anything, but I really doubt that has anything to do with the amount of posts you reported, or that they were lazy etc so thats why they didn't do anything. I don't know if I understood you correctly, but did you report whole threads, or posts inside?
I've used to report posts in bulk, usually several hundreds at once and I never noticed that mods decided not to do anything just because I reported a lot. I know that some users even reported several thousands of posts in a single day and it got deleted, so that's why I don't think that's the reason for your posts staying unhandled.
By the way, care to share some of those posts that stayed unhandled, if you can find them?