On thinner motherboards the mounting nuts at each corner of the back plate are too tall and will protrude above the top surface of the motherboard thereby reducing the clamping pressure. Once the standoffs are in place and the cooler is mounted, you can run out of threads just by going finger tight; there is a real lack of mounting pressure with these poor mounting designs.
I encountered the same problem with the Corsair Hydro H80i. I solved the problem by using #8 flat nylon washers/spacers that I picked up from Home Depot.
Retention nut out of the box:
Results in protrusion above the top surface of the motherboard resulting in loose mounting when the mounting posts are screwed into the mounting nuts:
Adding spacers (i.e. can be #8 flat nylon or metal washers) like as shown here:
Results in proper mounting nut height and results in proper clamping pressure: