Roland 300 double sheet detector

Hi I've got a problem with the electronic double sheet detector when running 80g offset.Even when it's set it will let doubles and triples through without stopping the press.
I've only encountered this on light stock but can play up a bit when backing up jobs with areas of print that travel over the sensor.
I've been told that this cannot happen as the other operators have not had this problem.the press is a 1999 model i think. I've done all the usual things like clean the glass.
Thanks for any input.

 

Double sheet detector must be set, with one sheet, for each material. Do you do this?

 

Yes.every time both mechanical detector and electronic.
The manual says either put a single sheet in the lays or set it when sheets are going through the press