Unfair insurance practice on shortfall insurance

I have been with outsurance for more than 7years, my vehicle was involved in an accident and outsurance declared the vehicle a write off, at first they agreed and evoked the shortfall insurance and now the insurance want to only settle for book value and they say the shortfall insurance would not apply bcos the vehicle had extras from the dealer, I have no idea why shortfall insurance cannot pay the difference and I have been paying for it all along, the vehicle is standard from the day I bought from BMW