"Conversions are neither created nor destroyed, they are absorbed by the Uniquest offer"
Many times we hear a Google Adwords campaign which were giving great result in past have stopped or nearly stopped giving return over time period. Most of the time search campaign shows this syndrome. It is not due to any technical reason, but a reasonable outcome of competition & their counter strategy. Every time a new Ad enters a Page, they replace someone already there, you notice it after a long time when you are knocked out of the competition. Let us analyze how intruders trounce the static ads on a search result page.
Most of the ads which gives good returns on a page to its advertiser are static for a reasonable long period. They inspire others to write around its own theme. Gradually, it creates competitors on the page, which start diverting clicks & conversions towards themselves. So, the ad which was unique once looses its uniqueness, and thus attention, clicks, CTR & conversion. It is not that it happens with intentional theme copying alone. Even if people does not copy your ad/theme, they happen involuntarily, much like Charles Darwin's Natural Selection Mechanism. You, consider a keyword rewarding because, you are meeting the users demand on the page, so you maintain it for a long time. Naturally, other advertisers too find keywords rewarding or not rewarding and accordingly they too tweak their campaign & Ad, and only those who were fortunate to devise an idea around a winning ad either because he studied or because he struck it by chance will find it rewarding and hence would stay on the same page. So, the ad which was great on a page, is great only till it is unique. Once, it creates clutter around itself with most Ads saying the same thing, attention of users diverts to another offer/theme, and other ads start facing gradual and steady Zeroing Out syndrome.