Cotter pins are so cheap that it's worth filing them and binning the ones that don't work. Make sure you're putting them in from opposite sides (that is, when the cranks are level, one cotter pin has the nut facing skyward and the other has the nut facing the ground).
I got some NOS 1950s Sturmey-Archer cotter pins at a car boot sale, fitted them to my girlfriend's bike - they seem to be much higher quality than any modern ones I've bought.
Cotter pins are so cheap that it's worth filing them and binning the ones that don't work. Make sure you're putting them in from opposite sides (that is, when the cranks are level, one cotter pin has the nut facing skyward and the other has the nut facing the ground).
I got some NOS 1950s Sturmey-Archer cotter pins at a car boot sale, fitted them to my girlfriend's bike - they seem to be much higher quality than any modern ones I've bought.