I assume you mean the SecuRemote VPN client in this case.
Does your Encryption Domain on the gateway include Network B?
If it does not, this won't work.
This VPN domain is defined in the relevant gateway object in SmartConsole under Network Management > VPN Domain.
If you're using the "All IP Addresses behind Gateway Based on Topology Information" option, then the Topology settings for your internal interface must be set to an option that includes Network A and B.
Policy must be reinstalled for these changes to take effect.