Exchange losing edits to appointments in a shared calendar

We have a situation where we have a couple of shared calendars in the public folders.  I am getting complaints that users are making changes to exisiting appointments and that these changes are sometimes lost the next day.  The changes do not seem to ever get lost the same day, so it seems to be something that happens overnight.  This is not all changes, just some, and with no apparant pattern.  

I am also getting complaints from a few users about not being able to see edits to appointments that other users can see.  I have not had a chance to investigat this one deeper, so I am not sure if it is related, but I thought I had better mention it just in case.  

On the calendars in question, I have gone into ESM and set the permissions to publishing editor for anonymous, just to rule permissions out.  

What suggestions can you give me on how to track this issue down?  I don't see anything in the event logs, but I am not sure what to look for.  

Thanks!