The most accurate response is I don't know! There is no one size fits all solution for advertising, I would need to review your Google ad campaign in order to determine the specific reasons for decreased results. In general, the effectiveness of a Google ad will decrease over time which is why it is important to be constantly adjusting your campaign. We are always tweaking our ad campaigns and split testing to produce better results.
It is very important to proactively manage any ad campaigns because paid ads can drain your marketing budget quickly with low results if they are left on cruise control. Paid ads are not a set it and forget it plan.