Computer playing hide and seek on network- sometimes there

If you do a major update then the pc is a brand new pc on the network and lots of settings will revert to default

It is sorted. New machine was on a Microsoft account for my log in. Older PC a local account. Making new PC log in to my user account local seems to have sorted the problem. Suddenly sharing was back. This was never a problem before. I've been network sharing files between PCs for years, and an assortment of PCs with mixtures of local and MS account log ins. The fact that the block became both ways only after updating Windows in the older machine suggests that they changed something.
All shares that I want are back. Strangely the new PC's icon still doesn't appear in the old one's network places, but the path to the PC and folders \\NEWPCNAME works fine. And mapped folders all appear as required. Which is good enough.
Thank you to all those who tried to help. Let's hope that this method will be the one that works for others when all else fails ( or even before- it's an easy fix).
Hope this didn't publish twice. Clicked "solved" and my reply vanished.