Prolimatech Megahalems 115X Mounting Kit Issue

[maklabouhlel]

I'm having an issue installing the mounting kit for a Prolimatech Megahalems cpu cooler on a socket 1151 board. It was one of the earlier ones that did not come with a socket 115X mounting kit so I purchased this kit from OcUK. When I install the backplate and try to secure it with screws, the backplate is still loose even after I've fully tightened the screws. I could probably get round it by placing washers but this shouldn't be necessary. The package says for socket 1156 so I assumed it would be ok for socket 1151. Any suggestions welcome.