Metric DIN 985 Nylon insert lock nuts are often called elastic stop nuts. This type of lock nuts has a nylon (usually a similar polymer and not actually nylon) ring inside of the nut. When you put the bolt through, it starts to thread the nylon, then it puts pressure on the screw and prevents it from vibrating loose or falling off. The nylon insert nut features an internal nylon ring, the addition of non-metal material makes the nut more sensitive to temperature and chemicals. This locking ring can be put into hex nuts, flange nuts, cap nuts, and wing nuts among others. These nuts should not be reused as the ring is damaged by design as part of the installation process.

Nylon-insert lock nuts aren't go all the way on. They're supposed to lock after one or two threads clear the locking ring; that's how they work. This is not a defect in the product. You can force them, but they'll fail, and the locking ring will pop out, or cause the bolt to break. They also make special waxed nylon insert lock nuts which are substantially more money which may allow them to go further than they are supposed to. You may wish to consider a k-lock nut or serrated flange lock nut instead, which is supposed to go all the way on.