its called a HANGFIRE its when the primer smolders for a brief time before igniting, after its hit by the firing pin, its frequently the result of penetrating oil contamination on the primer compound ,you should never get ammo covered in any spray oil, get the ammo soaked with some penetrating oils and it slows the ignition or renders the primer inert.
if your in the habit of spraying your gun down to prevent rust instead of proper dis-assembly cleaning and re-assembly it can and does happen, rarely with some products