The velour rooflinings on the later models have an inherent fault of "drooping".
This is because there is a thin layer of foam between the velour material and the cardboard backing. If you try to do anything with this foam in place, you will have to redo it @ a later date as the foam disintegrates with age.
What I do is to remove the rooflining completely through the boot, gently pull back half the covering (normally to the middle of the sunroof aperture, brush any foam off and then apply the spray adhesive, let it set for a couple of minutes until it is tacky, then gently roll the velour back into place using a plastic card or similar. This prevents any wrinkles or over-stretching of the material.
Leave it overnight, then refit to the car.