Treatment plateau is a common problem. Often, as already stated in other answers, this occurs as deeper ink goes "out of reach" of the treatment beam, and the highly organized light scatters and weakens as it passes through the tissue on its way to the ink. By the time it gets there, it might no longer be coherent. Another way of saying it's worthless. In response to this, an important strategy for the operator is to LOWER the treatment energy, where many would expect that the opposite is necessary. Less-intense light undergoes less scatter, in just the same way driving with low-beam headlights allows you to see deeper into a dense fog than using high beams. Erroneously increasing energy works very much against you and increases side effects. Tightly focused, high-energy beams can also elicit frosting on normal, un-tattooed skin, lulling one into believing the treatment is working, when it's not. With deep ink, it's all about the SNAP, not the frosting. If an audible snap is heard, however quiet it is, ink shattering is occurring. With lower energy, more passes can (and should) be made. Since there's no frosting, an optical clearing agent, such as a perfluorodecalin patch, would be unnecessary. Understanding these aspects of the physics and anatomy of stubborn tattoos, I have excellent success getting them over the finish line with my PicoSure laser, and your operator should be able to do the same. Good luck to you!