One of my sprite objects has bone movement as the behaviour, it was called "maindet" but I later decided to renamed it to "mainbone" then I made a new/different/fresh sprite called "maindet". This new "maindet" was being treated as the main bone piece (despite not even having the bone movement behaviour), it seems as if it didn't realise the the real main bone piece was renamed to "mainbone".
By the way, the bone thing will work properly again only once you go into the "edit bones" thingo - then click on some bones. It will then recognise the renamed main bone piece.