ping problem: Why one but not the other?

[Wm.G.McGrath <wgm@telus.net>]

Hi,

I've got two boxes (A&B) behind a third (C) acting as a firewall. 

All three boxes are working normally.
All three boxes can access each other on the local lan no problem.
I can use A normally and access the net through C no problem. 
But I cannot access the net from B through C.

/etc/hosts /etc/resolv.conf and /etc/host.conf all look normal on B.
Ipforwarding is enabled. 
Any suggestions on where to look? 
I really need to set up apt for the second box.
All input much appreciated,

	tia,
	bill