Ensure the glass is clean, rub a little spit on the suction cup, press the cup to the screen really firmly (you are trying to press out the air in the cup), then attach the unit to the suction cup mount, not before.
I know it is frustrating because my son had the same problem. This may sound a bit strange but I was told to use a very small amount of cooking oil on a finger. Encircle the edge of the cup and then try attaching it. By all means make sure it is just a very tiny drop just enough to coat the edge. You only want to seal the pores that exist in the vinyl suction cup and the glass. It worked for us. Hope it will help you.
By the way when I watch the mechanics change the oil in my car they always use a drop of oil to seal the gasket on the oil filter. I think it is the same principle.
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