The likely cause is that all the LAN networks are known to Router A and C, but not all the WAN networks. So it is also likely that static routing is used, and not dynamic.
It is possible that the two WAN links are in the same classful subnet, and if there is a dynamic routing protocol then it is auto summarising. This will stop the WAN network subnets propogating, but not the LANs. We need to see the config to confirm.
By default the Cisco router will use the outgoing interface as the source IP address. Try to use an extended ping on Router A. Just type PING and hit return. enter the LAN IP address of router C when asked, and ‘y’ when asked if extended parameters, then enter the IP address of the LAN interface on router A. Hit return to all the other questions. I suspect this will then work.
If you want the original ping to work, then add routes for the WAN IP address subnets, and that should then allow the PING to return.
If none of this works then apart from me not looking too clever, you can post the configs here, and we can have another look (it will need all 3 configs, but leave out the password information as it is not relevent.