Site to site VPN

The cli diag debug flow is your friend on a FGT. The  Packet-tracer on the cisco ASA is your friend. Run both of these and review the diagnostic output. You need route and fwpolicies and don't rule out any SNAT configurations.

Both of these  tools is your 1st line tool for trouble-shooting

Ken

Have a look at this cookbook http://cookbook.fortinet.com/ipsec-fortigate-cisco/ it describes the setups

You might also want to troubleshoot the packet flow through the FortiGate http://kb.fortinet.com/kb/viewContent.do?externalId=FD31702

Make sure your ip address is not being denied by the firewall policies

Please check below details,

On Cisco ASA:

1. Cisco anyconnect subnet must be in nonat acl.

2. Anyconnect subnet must be on Crypto route map acl with destination subnet of your frotigate site. 

On Fortigate:

1. Anyconnect Subnet is allowed in Phase 2.

2. The policy is configured on FortiGate to allow traffic.

3. A route is configured for anyconnect subnet.

Regards,

Deepak Kumar