[SlowToady]

Does all (or a significant portion of) your Google SE traffic arrive via AdWords? If so, you might want to ramp up your Search Engine Optimization efforts on the website itself. If you can put yourself on the first page of Google organically you should see more sales as well. You typically don't want a SE Campaign based solely on paid advertisements.

Google is constantly tweaking it's algorithms, so what worked in the past might not work now. However, the PageRank system is patented so you can take a read of that and try and deduce how to manipulate search results. Also, you can do some searches, you might stumble across tidbits of good information.

Keep in mind, Google isn't the only SE out there, and it's not the only one that counts. See how well you're doing in MSN and Yahoo, also (with organic search terms).

Sorry I couldn't help you more specifically with the AdWords problem, but I thought the above might help you, also.