Trying to figure things out a bit and maybe it could proove useful to someone else. So far, from what I've learned a low CTR can be caused by 1) Poorly placed adds 2)wrong colors 3) Wrong kind of ad for the website's format. It has no relation with trafic, but the kind of content may cause lower CTR. Is this correct so far? FerFAL
It could be due to any of the three reasons listed above and we can only help if we take a look at your site.
Actually I think it can have something to do with amount of traffic. Generally as my impressions have increased through time, my CTR has gone down. My suspicions for this is that I keep adding new, unique, useful content. So, for a wild example, if a visitor was looking at 10 pages before, that same visitor is looking at 15 or 20 now. Thus the CTR has gone down. If you like playing around with statistics and have access to the visitors metric, I suggest also looking at CTR in terms of Clicks per VISITOR instead of just Clicks per IMPRESSION.
It only has to do with your visitors and what they are really looking for. Placement, colors, blah blah is only a very small part. Read your log files. Load up your analytic software and figure it out.
Sites that actually have a lot of useful content probably tend to have a lower CTR because visitors are looking at more pages. Thus to really look at trends, it's the Clicks Per Visitor that may tell more.
Type of traffic is important. Traffic from some foreign countries usually earns less than US traffic.