VPN IPSec Tunnel is UP but unable to ping Terminal Server

Hello there,

I'm having trouble VPN with Fortigate to SonicWall since 2 weeks ago. The VPN Tunnel / Remote Gateway is up and pingable , but i'm unable to ping Terminal Server, it's request timed out all the time. The strange thing is when the ping from Remote Gateway is 260ms++ , Terminal Server can pingable . When the ping from Remote Gateway turn down to 216ms++ , Terminal Server is out.

Need your help!