The Enya #3 is the recommended plug for Enya four strokes. I have the best results #3s in my enyas.
Is it possible that you are using a higher nitro fuel, have deposits in the combustion chamber that increase the compression ratio, have an intake vacuum leak or are using WD40 as after run oil? These things will affect the longevity of your plugs. Have you readjusted the valves recently for wear, use armor all as a fuel additive or fuel with a rubber bulb? These also shorten plug life.
It sounds like a happy state then a progressively worse condition. It stands to reason that something is changing more and more over time. You then have to look for the thing that is effecting that change.