it could be a couple things
first thing you have to check is the GND ( ground)
but since you already said this wasn't it lets scratch that.
second thing can be the resistor unfortunatedly the only way you can test it is with an AMPMETTER wich will cost you like $200
or you can buy a new resistor holder kit for like $60
if the resistor is bad it won't charge the powder and there for won't stick to the part specialy second coats ( candy colors)
third is the pump
the old cooper pumps wear out with time and you slowly have to keep adjusting the flow and atomiser settings untill there comes a time when the settings are so high the same powder repels the new powder of the parts
you should have around 30psi atomiser and 20psi flow
forth and in a worst case it might be the multiplier wich will cost you like $1200 and is only sold by nordson them selfs
to me it sounds like a bad resistor
you can get the manual here
http://emanuals.nordson.com/finishing/index.htm
and you can buy parts here
www.azpowder.com/