Computer playing hide and seek on network- sometimes there

My WIN10 PC isn't allowing access to shares, but it can sometimes be seen in the network places on the remote machine. I've been through vast numbers of articles about "My PC doesn't show on the network" and followed the advice. Even changing bits of the registry and changing the PC's name.
But I can *sometimes* see it. But even then I can't connect to shares. It says I haven't got access.
Or it asks for my credentials first, and then says I haven't got access.
One suggestion that I've had was change the ip address (192.168.etc.).
That worked. All was fine, all my shares were available, everything seemed fixed-until I rebooted.
And, I keep setting the password protected sharing to "off" but it seems to turn back on again.
When I try to connect by entering \\PCname in the other PC it asks for credentials. When I enter credentials it says I haven't got permissions. This PC can see and access files on the other PC. It just can't always be seen and can't be accessed.
Windows 10 is up to date. 21H1. It's a new PC (Chillblast).
Windows Firewall seems OK to allow connections in. I've switched AV software to see if that helps.
I've now been round and round in circles trying all the different tips to solve this riddle. My head is spinning. And I don't know what to do next.
Help, any new ideas?

Even If your basic network is constructed properly... The Network browser will always act out in home networks.
But make sure you have.

All machines with disctinct names
All in the same Workgroup
All NetBT enabled
All same NLA level
All Client for microsoft network enabled
All File and printer Sharing support enabled

Create your shared Directory on the server Machine and set the security for access.

From client try to ping the machine (firewall disabled) even if it doesn't show up in network neighborhood.

If the Machine gives a response (firewall disabled),
And you have MachineName, ShareName, UserName, Password, to get there.
open an elevated cmd prompt. and map the drive for good.

Code:

net use X: \\MachineName\ShareName /user:UserName Password /savecred /p:yes /permanent

This should map your shared folder as drive X: on the client machine. Reboot see if it works. If this works try to re-enable the Firewalls and reboot again.

Samuria said:

Basic have you set the permission on the share to everyone and are the folders NTFS permission set the same as this is the most common reason for the error

Yes, permissions and sharing set to "Everyone".
And as I noted, when briefly, the IP had been changed sharing across the network was fine. Then it all stopped again after a reboot.

terryBernstein said:

Yes, permissions and sharing set to "Everyone".
And as I noted, when briefly, the IP had been changed sharing across the network was fine. Then it all stopped again after a reboot.

I never seem to have issues (knock on wood). Do you have same username and password on both computers? Mine match exactly and these are my settings if you want to match them.

Samuria said:

With all pca on you this on every pc does it see them all and which is the master browser as its that which resolves the pcs
NetBScanner - NetBIOS scanner

How do I resolve which is the "master browser"? That's not something I've come across but my experience and intuition tell me that maybe this is something I need to look at.
The Netbios scanner sees the other machine, the one that shows up on teh network, in the new machine' But not the new machine, which I actually ran netbios from. i.e. not only is my new PC not visible on the network to my old one, it's not even visible to itself, apparently

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tomdsr said:

I never seem to have issues (knock on wood). Do you have same username and password on both computers? Mine match exactly and these are my settings if you want to match them.

One machine logs in with a local account.
The other has two users, mine's admin, and I'm using a Microsoft account (This is the new one that is not visible).
BUT. When I set password protected to "off" it doesn't stay off.
I do wonder if 21H1 Windows has some damn fool Microsoft setting changed. The other PC btw has a slightly older version of Windows. (20 something.) But that's the one that is visible on the network. (I'm doing an update now, which I might regret).

Yes the best way to do that is to disable Browse master election on all machines except the server and leave it always on.