The answer would be Norton. Anytime you stray from stock you have issues. Also in a mix of wired and wireless you find it to be hit and miss so you are looking at the router's firewall too.
This is with Windows 10. I have a desktop connected wired to the router and a laptop connected wirelessly. The desktop sees the laptop but not vice versa.
I've done all the things mentioned on the web. All boxes on advanced sharing are checked correctly, all services enabled, Norton Security firewall set to full trust, SMB enabled.
Anything else I'm missing?