Did you "pre thread" all your holes?
To do this, take a screw and simply screw it down into each hole you intend to use.
This will put threads in the hole that the screw can follow when you stack up the footstrap/
K9-lok and washer (s).
Also, it's very important to "compress" the foostrap/K9lock and washers so the screw threads are
well clear of the K9lok so they can engage the threads you have "pre threaded" in the hole.
Hope this helps,