A connection from server 'c' to server 'a' that accesses a linked server that connects to server 'b' requires Kerberos to be setup and working correctly for all 3 servers.
This is known as the double-hop issue and most people end up changing the connection type between server 'a' and server 'b' to avoid the issue. If you map a user to a specific login/user from server 'a' to server 'b' using SQL authentication - the second hop is no longer windows based so it doesn't need Kerberos.