Session intermixing issue - due to Cookie not being sent every call?

when HTTP_REQUEST {
    Determine the persistence cookie name.  
    This assumes it hasn't been customized in the persistence profile.
    If it has been customized, just hardcode the name here instead.
   set persist_cookie_name BIGipServer[LB::server pool]
    Save persistence cookie value if it exists in the request
   if {[HTTP::cookie exists $persist_cookie_name]} {
       If the persistence cookie is present in the request, save the value
      set old_cookie_value [HTTP::cookie value BIGipServer$pool_name]
   } else {
       If the cookie wasn't present in this HTTP request, unset the variable so it's not available for subsequent requests on the same TCP connection.
      unset old_cookie_value
   }
}
when HTTP_RESPONSE {
    If the response doesn't already have a persistence cookie and the request did, insert the value from the request
   if {not ([HTTP::cookie exists $persist_cookie_name]) and [info exists old_cookie_value]}{
      HTTP::cookie insert name $persist_cookie_name value $old_cookie_value
   }
}