Over the past five months our Google Ads campaign has had a significant increase in CTR and decrease in conversions. After analyzing the last 18 months of web logs, we found a massive increase in visitors that do not download any website resources, which only occurred in the last five months. For these types of visits, there is only a single entry in the web log with the referring URL of google.com and a URL with the gclid (google ad click ID). There are no web log entries for downloading the stylesheets, javascript, images, fonts etc.

We have had about 10,000 of these types of visits over the last five months, compared to about 250 for the preceding 12 months. This represents about a quarter of our total ad clicks (40,000) over the five month period. The IP addresses for these 10,000 visits are unique over the last 18 months of web logs and none of these visits converted to a sale. Our conversion rate over the preceding 2 years was over 40% on average. Almost all of these visits are from mobile devices with iPhone as the User Agent.

We understand that a returning visitor would not download website resources, such as stylesheets, if they are already in their browser's cache. However, it seems unlikely that we have received 10,000 clicks from genuine returning visitors with different, unique IPs, and none of them returned to purchase anything.

Is there any other reason why a visitor/browser would not download website resources? Is this most likely the result of a click farm that simply swithces the IP address of mobile devices, but does not clear the cache?

Any suggestions or advice would be greatly appreciated.