about snat pool member IP address select(web server access unusual)

hi,dear friends,i have two servers , snat pool have two ip address, when i access the VS,after snat to access the pool member,login the web sites,operation others items,the web site let me log out,i use source address persitense,and i found the source address persistense is successfull,only LB one server,if i only use one snat pool ip address,there is no problem like this,what's the reason??someone can solve the snat transaction problem????