A drop in conversions can be caused by many factors. The folks over at Google and industry best practices suggest you try at least the following to troubleshoot your account to identify the reasoning behind the drop in conversions:
Have you recently made changes to your website?
Have changes recently been made to your website? If so, a new unfamiliar or unclear design, broken links or just a lack of conversion tracking code could be causing your drop in conversions. You should check to make sure the correct tracking codes are correctly updated on each page you need to track.
Have changes recently been made to your pay per click account?
Budget and bid changes, adding or removing ad text and keywords, pausing or resuming ad groups/campaigns can all cause negative changes within your sites conversion volume. Such changes can affect conversion numbers by lowering average position and ad visibility. To minimize the effects of changes and to understand what changes caused which reactions, it can be best to limit the type and number of changes you are making at once. If multiple people are working within your Pay Per Click account – Google makes it easy to run a change history report.
Is this a historical trend that occurs?
Run a daily account or campaign level report for the last thirty days to understand if there is a daily/weekly/monthly trend that occurs within your account. Do you find that your sales numbers usually drop historically this time of year? If you don’t have this type of information available you can also check out Google Insights for Search. It is a great tool to see trends in user generated searches – like this one I did for Ground Hog Day. Not too common of a search this time of year and just as expected the graph shows this trend.
Finally, it is important to note that conversions can take up to 24 hours to be reported within many of the search engine interfaces. Therefore, conversions may appear today for clicks that were made yesterday.