Good day,
What kind of NFS server are you using? What happened around the time this problem appeared?
If the files exist in the directory and each host has network access to the NFS server, it may be that the files' attributes changed and they're no longer visible to certain hosts. Try to list the hidden files in the datastore with ls -a. You can also try uploading a test file to the datastore from one of the affected hosts to see if you can access it. This can give some clues as to which files it affects or perhaps the permissions on the entire export.
Cheers,
Mike Brown