Cisco Firewall :: 5520 - ASA Phone Proxy After Failover?

I have a problem with my asa phone proxy. i have two ASA 5520  in HA. I have 10 phone register with ASA active primary. if i execute the command show phone-proxy secure-session. i can see the phone session on the ASA.
 
if i perform the same command on the passive ASA i can't see the session replicated from the active member.
 
If i switch the cluster the phone enter in a registrating loop and can't connect to the ASA now active.
 
If i switch back immediately (the session are still present on the first asa) the phone register again and all works

the ASA have version 8.4(5)

the phone are a 7921g
 
is normal that the skinny don't start again and re-register the phone on the ASA that became active after failover?