Hi Craig,
Is there a node or pool member-specific configuration for the member(s) you're seeing this on? It sounds like the member might have monitoring set to none either on the pool member or maybe the node definition.
Can you configure the simple TCP monitor on the pool? Then please post an anonymized copy of your pool definition using 'b pool POOL_NAME list' and the status using 'b pool POOL_NAME show'.
Also, I added a wiki page on some tips for troubleshooting monitors. You might find something useful there. Or if you figure out another method that works for you, feel free to add it.
Troubleshooting LTM monitors
http://devcentral.f5.com/Wiki/default.aspx/AdvDesignConfig/TroubleshootingLtmMonitors.html
Thanks,
Aaron