Cookie persistence not being maintained if server doesn't send cookie back.

Do you have "Always Send Cookie" checked? (The default is unchecked.) If unchecked, the BIG-IP system will NOT send the cookie out on each HTTP response. (It's more efficient that way since the cookie is still on the client.) However, since you are specifying an expiration time for the cookie, it means the cookie will expire in 3 minutes, no matter how many times the client sends the cookie to the BIG-IP system on a subsequent HTTP request, as the BIG-IP system never sends an updated cookie with a new expiration time. After 3 minutes, no more persisting; load balance instead, then a new cookie with new 3 minute persistence expiration. If you want the 3 minute expiration to be a sliding 3 minute window - one that gets refreshed on every subsequent HTTP request to the same VIP - then make sure "Always Send Cookie" is checked (enabled).