Hi
check this link and try to use the troubleshooting commands in the link.
http://www.cisco.com/en/US/products/ps6120/products_tech_note09186a00807e0aca.shtml
> NAT-Traversal or NAT-T allows VPN traffic to pass through NAT or PAT devices, such as a Linksys SOHO router. If NAT-T is not enabled, VPN Client users often appear to connect to the PIX or ASA without a problem, but they are unable to access the internal network behind the security appliance.
Note: With IOS 12.2(13)T and later, NAT-T is enabled by default in IOS.
So, If you have an IOS lower than 12.2(13) try to enable the command or upgrade your IOS version.
Regards
amady