Site to site VPN connect, but not able to ping any other side IP [Closed]


I had build a VPN between Fortigate 80C and TPlink 604.
In my test environment, everything working fine.
When I bring TPlink to customer's office , everything wrong.
I can connect VPN. On Fortigate, I found that only outgoing data. No incoming data.
On Tplink, VPN connected.
I can not ping other side computers.
I still have a same TPlink 604 in my office. Only change remote IP address on Fortigate IPSEC setup then I got VPN connection. I can ping each other right now.
I don't know why. Only different is the ISP.