Hi Kevin, thanks for the quick answer.
Well, that was an easy fix !
As far as I understood it is necessary to "rig" every device to the truss in the same height (z-axis), is that, what You meant ? Any way it works fine in the theatre excercise.
And, yes, snapping to object was selected, but i guess that the snapping wouldn?t work on the z-axis if the Objects were 20' away from each other ?
Thank You again for Your help, Im keen on seeing if that also solves my 3D-problem on my "real" project in LA
Frank