Standby Has Fewer Online VIPs Than Active – Requires Manual Monitor Reset

Which one out of the active and standby device is generally showing more online VIPs? Do you notice any patterns in terms of a particular pool or pool member(s) failing their health checks more often than others? Are you using the same nodes across a large number pools? Do you have health monitors applied also at the node level or is it just at the pool / pool member level?

I would recommend checking out the following article which has good tips on how to troubleshoot health monitors:
Troubleshooting health monitors

From past experience, I have had cases where the issue with health check failures were attributed to the back-end pool member and other cases where it was the BIG-IP. The BIG-IP by default uses the "bigd" daemon to send health check probes on the control plane. If you have a significant number of pools with health check monitors applied, then it could be getting overwhelmed. As per the previously mentioned article, you can check the memory usage of the "bigd" daemon by running the following command:

ps aux | grep bigd

If you notice that the memory usage for this daemon is high, you may want to consider switching to "in-TMM" monitoring which makes the BIG-IP send health check probes using TMM (data plane) instead of the bigd (control plane)

More information about in-TMM monitoring here