Hi mcdave, I'm new to McAfee Communities but I might be able to give you some things to think about. My first thought was to encourage you to look at your policies for the endpoints that are not using the AH's anymore. Specifically it would be the McAfee Agent policies. Perhaps there was an accidental change that pointed them somewhere else (aka modified their sitelist.xml). Best way to verify this is to remote into one of the systems and do an Update with the McAfee Agent Status Monitor up... watch what system the endpoint is looking to to get its update. Of course, you can also watch the log from ePO.
Whatever the case a copy of that log to this discussion might prove helpful.
I'm not 100% sure about your last question. My gut tells me you have to reinstall it but I may be wrong.
Sorry for the delayed reply (official holidays )
One week later (after restarting all the handlers a week ago) there is a very very very little amount of client reporting via agent handlers but it's faaaaaar to less in the past it was equally spread (25% per handler)
As you can see we use the default assignment rule that usses all agent handlers (and yes all handlers a online as you can see )