I have also had issues. I found the cotter pins vary and that was my problem. One company supplies the pin with a metric thread and imperial hex nut! They said it was to make the threaded part stronger which I guess it does. Also the threaded part of the cotter pin is offset to the body of the pin. I ended getting my cotter pin from George Prew and it worked for me. Good luck and hope 2017 is a good year.
I have three new cotter pins all sold as 3/8" and each one is different in terms of the cut out, threads and nuts and, yes, one has the threaded portion offset.
However, they all work on my non-folding kickstart but all fall through with the new folding one.
I heard back from Steve at Feked this morning and he has asked me to send back the kickstart for checking. I will let you know what transpires!
Brian