Hello Flashback,
Thanks for the continued interest in Hauptwerk.
In case it helps, my general suggestions would be to make sure that each of your keyboards has its dedicated power supply unit attached and powered up, and to to try connecting all of the keyboards to USB ports directly on your Mac, or via a good-quality (e.g. Belkin) *powered* (not 'bus-powered') USB hub 2.0 hub.
Also note that all of the MIDI keyboards/devices need to be connected and powered up before you launch Hauptwerk, and to remain so whilst you're running it; Hauptwerk doesn't support 'hot-plugging' devices.