prop nut

Hello All, Can someone explain why on two of the motors the prop nuts screw on partially and on the other two they don't screw on? They are brand new motors and brand new prop nuts that I made sure are the correct size. (M5) My quad is an eachine wizard x220 that is new. These are the prop nuts...

I have just completed building my first quad and I have used self-tightening bolts on the motor. However, now I need to change the first prop, but I cannot get the bolt loose. Does anyone have any tips to get the bolt loose (and I have double checked which way to loosen the bolt).

Is there a trick to keeping the little buggers from flying off into never never land??? I tighten them down and they always work loose. have spares on order since it looks like i'll be needing them. tired of replacing broken arms & props too!