I suggest you to have a look at the Admin guide .. there is achpater about configuring fail over. Make sure you configured LAN failover and also state ful failover.
http://www.cisco.com/en/US/products/ps6120/products_configuration_guide_chapter09186a008054c4b7.html
Basically on the Primary PIX you need :
failover
failover lan unit primary
failover lan interface failover
failover replication http
failover link failover
failover interface ip failover 10.10.10.9 255.255.255.252 standby 10.10.10.10
On the secondary failover you need:
failover
failover lan unit secondary
failover lan interface failover
failover interface ip failover 10.10.10.9 255.255.255.252 standby 10.10.10.10
NOTE: You can also use the optional command failover key to encrypt communication between the PIXes. This is recommended when your PIX is terminating VPN tunnels as the the usernames, passwords, shared keys ..etc will replicate on clear text otherwise.
I hope it helps .. please rate if it does !!!