thumbsup80 - 4:22 pm on Jul 11, 2013 (gmt 0)
I assume you did not make any major changes in your account. So, I would check my avg. position compared to weeks where your CTR was higher. There might be a new competition bidding higher than your max cpc which might have lowered your position and your ctr accordingly.
You can also check auction insights which is a new feature from adwords to see your competitors and what is and what was your impression share.
Comparing data is the best thing you can do to figure out and adwords is getting better in the field day by day.