Does anyone else have this problem in rollercoaster tycoon 3?

Okay, so my park is a very large park, I have 250 janitors and they're all assigned to paths and such.My park stays clean.I mean the most litter I had was -12 but it was cleaned up right way.I close my park down when its time to clean, so i closed it down and it says -12 litter.So I figured I wait and let them clean, they usually clean it all up.But this time their not cleaning anything its stuck at -12 and I looked around my park and I don't see any vomit or litter.