Hugh,
I'm going with 1 x Plaggy enforcer controls & NOTHING as wired devices on the system ?
Supervision fault, what's the signal strength of each radio device on the system ?
Are all batteries 2yrs old or less, when they are replace spin them around in the holder to ensure there are go contacts
If the house is not 'kin massive & the controls are no in silly place eg in cupboard surrounded by display cases of lead painted figures or something
Swap the device far away for an identical item which works fine in a different location, ideally the item which drops of in location A should drop on in location B, proving the device is duff... chances are all the hardware is fine & its an issue with the environment of the installation ?