I have a 300 Whisper that the Thompson Custom Shop built for me. I don't reload anymore so rely on factory ammo or a friend that reloads. I bought some subsonic 240 grain sierra matchking bullets from Corbon. I also bought some corbon 125 and 150 grain sonic loads. I bought the 240 grains to head shoot suburban deer quietly. THose subsonics are as quiet as a 22 short buttttttttttttt so far,,,,,,, every other bullet doesn't go off. The 125s and the 150s from Corbon and the 150's my pal loaded all work fine.
Its just those blankety blank 240 subsonics that fire about half the time. I know I got some junk shells from Corbon. They had them on a half price sale.(I wonder why).
Try another brand of ammo. I hope it was just a problem with those Hornady bullets.