Permissions problems, keep changing and can't save from within application

Hi there, we're running Mavericks server on our local network with a half dozen MBP and iMacs connected.

For a wee while now, our users have been getting permissions problems. Documents (Excel documents, InDesign, Photoshop etc.) are becoming read only and we can't save them to either the same folder from within the application. Even ones that the same user has created. The thing is, we can copy the document to our desktop and then back again to the same folder and continue to edit and save normally.

I've tried setting the Access for our "Group" to "Read & Write" but to no avail, this keeps happening. I can login to the server as admin and correct the permissions but they seem to keep reverting somewhere along the line...

Apologies if there's something obvious I'm overlooking.

Any recommendations? Would you recommend just removing all users and groups and starting again? Am I missing something?