As has been said -
Make sure they are terminated correctly.
Conductors snap leaving the insulation holding the wire in place or
wires are pushed into the terminal too far so that screw only contacts with the insulation.
OK so you think it's to do with connections. I was looking for someone to say yes or no if the wiring in theory was correct. If it was then yeah I can mess about with the connections. But didn't want to do that if the wiring set up is wrong........ cos it's a killer holding these bad boys up above your head while fiddling about with these little screws, as I'm sure you well know!