Diagnose & fix why google index is dropping

Our Google index crawl is steadily dropping since November 17th. We need someone to analyze why this is happening & correct the problem. We are a 10 year old ecommerce website with good google rankings. We have not purchased any link building or seo scheme & we will assume that therefore there is no google penalty In October, we have installed an on page seo extension in our magento store which requests google to not crawl any of our duplicate product pages. We are concerned because our sales are dropping. Please Private message for website name so that you may check us out before submitting a bid