Having issue on LTM

Going thorough some issue on F5 LTM

 

 

1. When both primary and backup server are up all connections are directed to primary

 

Once primary is down all traffic diverts to backup server (working as excepted). Once primary server is back up all new users connection still get diverted to backup

 

 

2. After about 20 min of primary server coming back up we notice that all connection get bounce from the backup server and user are required to log back on at this instance all connection are back to primary ( if session persistence is working all user connected to the backup server should stick to this server unless they log off themselves or backup server is down. ).

 

 

Can you help on this...