There are a lot of things that can work but slowing down when you're lining should be half the effort. Thinner inks can also make a difference, I know by experience that with certain skin types Eternal lining black can be harder to get in. Tougher skin basically means it needs more force to be punctured, so changing just one of your variables can make a big difference. Think ink, needles, machine setup or "skin prep". I personally would chose to change a variable you control. If you think preparing the skin would do the trick for you instead of changing your setup, try using warm water in your squeeze bottle and/or massaging the skin more while your working.