2007 trx450r valve problems

I just shimmed my valves a month and a half ago, I rode it a ton, but not enough that think i would need to adjust my valves again. I did that last night. I keep my air filter clean after every ride, so anyone know why they went out so quick?

Second question is that I am on my last shim so i cannot go any lower. After it happens again what all does the job entail? I know ill need new valves and springs but is any head work neccessary?