\\ UNC path vs. DFSN path

so on the LAN we use \\domain.local\namespace for my docs and a shared folder. My docs are made available off line, shared docs are not.

servers are 2012r2 and 2016 and clients are windows 7 to windows 10.

Problem:

occasionally we gets complaints from users who can't use DFSN path to the shared folder, they have to use the UNC path i.e.,\\servername\share etc. We see this on the LAN and over VPN and haven't been able to figure it out.

Any ideas?