There is also a great book by one of the Eisenberg Brothers, oops! Call to Action! I think! Ill Post the link later!
Hope this helps!
Other possibilities might include:
* The ad is not particularly compelling (as steaprok is basically saying in a different way, I think.)
* There is something about the ad that is actually 'turning off' potential customers. Relatively common examples of this might be poor grammar, misspelled words, awkward phrases, uncommon abbreviations, and so on.
It's very important to remember that when a user sees your ad, they know absolutely nothing about your site. And, the entirely of what they know about your business is what the see in your ad - so really make it sing.
Others on this forum could probably tell you endless stories about this, but I've heard of many cases in which a single word in an ad being changed resulted in a dramatically different CTR. So I'd advise continually testing multiple ads within ad groups. Maybe start with two ads, let them run for a while, pick the best one and then test that ad against another new ad. Rinse and repeat. ;)
AWA
Important factor to think about in regards of QS is this one: While your efforts in improving QS may result in seeing your QS getting better, that should positively affect your CPC, not your CTR.
As AWA said, check ad text, test it. Since you’ve been focused on your CTR, keep in mind that your landing page has nothing to do with it directly. Indirectly, with better QS, you may achieve higher ad position with same max bid.