I have had this before a few times--have them use cocoa butter on the skin for at least a few days before tattooing,also massage the skin a bit and try to get to get the client drink plenty of water for a few days. Some people have really tough skin due to a lot of outside factors-tanning etc. Put more pressure on the needle without getting crazy. Make sure you do a really good stretch.Dont use to thick of pigment. Slow down your hand speed. Some people have large open pored skin and some have very small tight netted skin -look close at it under a light and you will see. Think of skin as a basket weave- a tight weaved one is hard to poke thru as and the loose weave is easy-a standard super long taper will work better as it will slide in easy but is still strong enough to hold a lasting point.Also switch out your needles after a hour are two as this type of skin can really blunt them quick(which works great for soft skin-like a bullet taper).
Also you might want to switch to a really hard slappy slow running coil-I have one I pull out for skin like this or very large mags-the thing with rotarys is when you use large sizes are have really tough skin you must do the popping in/push as a hard hitting coil slow coil slaps it/pops it in past the outside touch layer.
Hope this helps!