OS X Server: why would some Win 7 clients not be able to open a file, but another can?

I have an OS X server (build 4, on Yosemite 10.10.1) serving a mix of Macs and PCs. The Macs can all access and interact with the server fine. The PCs - they are all Win 7 SP1 - get mixed results.

Most of the PCs can not open files. They can write/create new folders and files, and delete folders/files, but when they try to open files they get an error about the file being in use or "read only".

One PC on the network CAN open files.

The thing I dont understand is that all of them are in the same user group on the server and have the same permissions. What would allow one PC to work and the other PCs to get a read-only error?

Facts:

all users belong to "allstaff" group

all PCs can see and map the drives, navigate the sharepoints, and create/delete files

all Macs have full access

all PCs except one get a "read only" or "file in use" error on any file on the server

one PC in the office has full access to the server

Sharepoint permissions:

drwxrwx---@ 110 admin allstaff 3740 Jan 1 11:18 documents/ 0: group:allstaff allow list,add_file,search,delete,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit