Some bits here are guesses
A machine will not necessarily let another machine see the files on it at all.
Under network and sharing center, go to advanced sharing and let it share with guest or public machines.
I believe this allows other machines to initiate sharing, but some sort of permissions may still prevent sharing - even if sharing for everyone has full control.
On the machine trying to access the files on the remote machine you can have it use credentials to get access.
Go to Credential Manager -> Windows Credentials
Add the remote machine by ip name or ip number.
Add an account and password that has permissions on the remote machine.
For some minimal level of security, make an account on the remote file machine for file sharing.
Only give it access to the shared directory.
Use that accounts credential in Credential Manager.
