Cannot connect to Asterisk over VPN

I have two sites, site A has my Asterisk on I have a VPN using Fortigate 60D firewalls through to site B Everything works perfect through the VPN to each site except. But if I go over to site B I cannot ping the pbx and if I take over an extension that works in site A it will not connect to my Asterisk box in site A. I have entered the boot info in dhch so it should be working fine. But like I said it won’t ping or pull the config from the pbx. I’ve added site B lan to the Asterisk sip menu, but with no joy.

Please help

Any help would be most appreciate. Many thanks