Source address persistence issue

we have a VIP configured with 2 Pool members and have configured source IP persistence. Load is always going to one pool member after another pool member flap for a while.

 

 

example : two pool member are server3 and server4. and load is not going to server4 after server4 monitored down and came up after 1 minute.

 

 

To correct this i need to reset the persistence from source IP[User IP] using command bigpipe persist client delete . This is only a workaround. can anyone suggest a permanent solution??