Weird network problems

I'm so lost on my network problems they seem to make no sense.

I have 4 computers connected via a Dlink Router and my Cable model connected to it as well.

All computers have internet and their own IPs.
Lets start with just 3 of them to keep things simple.
Call them A B and C

A can access 1 shared folder on C but not the others. A can't access anything on B.

B sees its own shared files and computer 4 were not talking about right now.

C sees nothing at all.

They all have the same Subnet Mask 255.255.255.0 and the same Default Gateway 192.168.0.1

Pinging...
A can ping B via IP and Computer name
A can ping C via IP but not using the Computer name.

B can ping A via both IP and computer name.
B can ping C via IP but not computer name.


C can ping A via IP and computer name.
C can ping B via IP and computer name.

Each computer has shared folders and they did work fine but now have stopped.

I don't know what else to do other than pull my hair out.

Thanks

 

One more thing running \\computer A and \\computer b seem to work but still can't see them in my network places.

\\computer A from computer C gives. "No network provider accepted the given network path"

\\computer C from computer A says nothing and never pops up with a window or error or anything neither does putting in the IP