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.

Way back in the day when trying to rank a website at the top of the Google search results, many people would identify key words that someone is likely to search then use these words many times on the page.  Sometimes website developers would even add keywords back to back in an attempt to use the key words many times.  Google is well aware of these tricks.
Today SEO (search engine optimization) is much more complicated than just typing your keywords as many times as possible. Actually, if you type keywords too many times or type the words in a way that doesn't make sense, Google may reduce your ranking on the search results. Keywords alone will not get you to the top of the results, you will need to make sure you are developing good relationships with our websites, getting backlinks, and having citations on multiple websites are a good start.  Getting your website on the first page of search results and keeping it there can be a full time job.