Clients in LAN can't see each other

Question

We have 2 windows 10 PCs sitting next to each other plugged into the same router (Fritz!Box). They are both on the same network mask 255.255.255.0 and both clients have the windows firewall disabled. Client A cannot see client B however and it's inexplicable because client C can see both and both can see client C. We have no isolation in place on FritzBox that we know of.

Client A has IP: 192.168.178.73 Client B has IP: 192.168.178.29 Client C has IP: 192.168.178.71

Issues:

1. Client A cannot ping Client B (but sees it in ARP)
2. Client B can ping Client A
3. Client C can ping both A and B and A and B can ping C
4. No matter who pings (ping -t) client A, A does not show any incoming connection in netstat

Questions:

• How can we ensure that all clients (LAN party) see each other?

• Netstat won't show pings so how can we watch incoming pings/connections on Client B?