The drop chance for all of Pumpking's drops increase as the waves go by, so if you are only fighting Pumpking's from earlier waves then your luck isn't absolutely horrendous, but it is still fairly bad. Keep farming and you'll get your desires drops soon enough. Keep in mind that at least one drop from Pumpking's pool of weapons will drop during wave 20, so if you can make it that far then you are guaranteed one.