The short answer
Why laser struggles with grey and white hair
Laser hair removal works by heating the pigment in the hair. Grey and white hair has little or no pigment, so the laser has nothing to target. Electrolysis uses a fine probe and a small current in each follicle, so hair colour does not matter.
That is why a laser that once worked can stop working as hair turns grey. The grey hairs are simply invisible to it.

