I can not see one of my Pis from the others

I have a “nest” of three Pis. Pi A can see and write files to B and C. B and C can see and write files to each other but not A. (They could at one point.)

Historically I used NFS for them all to communicate. I have uninstalled and reinstalled both server and client. No change.

Any ideas will be greatly appreciated.

Are you able to share some error messages?

I can not ping it from any other Pi nor mount a drive. All of its web services are available.

I apologize, as I am not sure where to look for error messages or which logs to look at.

Do you have any firewall or similar running on the system not being reachable? Maybe a VPN?