Maybe if I explained how condensation worked would help you understand this better.
The amount of water that can be stored in the air as water vapour depends on the ambient temperature of that air.
By putting insulation down, you have lowered the temperature immediately above the insulation layer, because more of the heat is now retained in the house - where you want it. However the same amount of water vapour from the house is still getting through the porous plasterboard and insulation and making its way into the loftspace - that's what it is damper.
Your only practical choice is to increase the air flow in there, to help drive out the water-laden air.