Workgroup works yet doesn't

I have 3 pc's in my LAN
Mark= Win7 32 bit
Dave= Win10 32 bit
HTPC = Win10 64 bit

I'm trying to share the files on HTPC and have created a workgroup.
Dave and Mark can share files
Mark and HTPC can share files

But Dave who can connect to the win7 pc through the workgroup can't connect to HTPC
Error is
Windows can't communicate with the device or resource (HTPC). The computer or service you are trying to reach might be temporarily unavailable. If the device or resource is nearby and you have access to it, make sure it is turned on and connected to the network.

Since the win7 pc can connect to the win10 HTPC why can't the win10 pc?

What net use command run on which of the 3 pc's.
if the win7 pc can connect how could the workgroup by set differently ?

I used this microsoft site Net use to read up about the "net use" comman before I used it.
I also used this site http://pcsupport.about.com/od/comman...se-command.htm to get a better understanding of the net use switches.

There are about 13 switches with this command and some can be used in conjunction with each other give ove a hundred possible combination. I'm not a mind reader , you need to be more explicit in what it is that you want me to try.

A simple example would save us both time.

Workgrops ARE the same name.
I have changed the passwod to make sure both are useing the same password.
HTPC can see Dave in windows explorer and can see and connect to Dave's shared folder.
Dave can see HTPC in windows explorer under workgroups but cannot connect.
Troubleshoot control panel when run on Dave sees no error in workgroup, folder sharing or network diagnostic