SHORT ANSWER
If your practice skin isn't taking ink, the most common causes are: (1) needle depth is too shallow, (2) the ink is too thin for the material, (3) your machine speed is too high, or (4) the surface has a coating or residue that repels ink. Try going slightly deeper, slowing your machine, wiping the surface with alcohol first, and testing a different ink brand.
Check your depth
On practice skin, you usually need to go slightly deeper than you would on real skin because there's no dermis layer to hold the ink. If the ink wipes off completely, you're not going deep enough.
Check your ink
Some inks are formulated for real skin and don't bond well with silicone or rubber compounds. If one ink brand doesn't work, try another. Thicker, more heavily pigmented inks tend to work better on practice surfaces.
Check your speed
Running your machine too fast on practice skin can cause the needle to skip rather than deposit ink. Slow down and let the needle dwell slightly longer at each puncture point.
Clean the surface first
Some practice skins ship with a light residue or release coating from manufacturing. Wipe the surface with isopropyl alcohol before your first session to remove any film that might repel ink.
Accept the limitations
Practice skin will never hold ink the way real skin does. The goal isn't perfect ink saturation — it's consistent hand movement and line control. If your lines are clean and consistent, the pad is doing its job even if the ink isn't perfectly deposited.