Training. There are quality differences in nuts. I put them on three of my trailers and an ice cream truck. I broke six bolts trying to change the washers. see pic - I bought good quality Dorman Nuts from O'Reilly's for $2.99 each and will be using them from now on. cheap nuts are softer stuff in my opinion and I find the threads break easily. See photos with broken threads. not quite sure what is causing this. Of course, I may have overtightened them, but that's what you get with cheap nuts. Some of them just snap my hairpin threads because they are corroded for some reason. Either way, it's just not worth saving money to switch from cheap nuts to good nuts.