Some headlights will yellow quicker than others depending on the stage of oxidation. A trick is to use sparr urethane, mixed 50/50 with mineral spirits, then applied and left to smooth out. Hell all you have to do is wetsand the light down to new plastic, not even having to polish because it allows the urethane to sit.

Or you can also seal the headlights after polishing with a polishing compound with a product called optimum opti-coat which adds another layer to the light (its a permanent coating as opposed to wax or sealant.)