Possible reasons:
1) People are popping in and out of the advertisers sites, reading about each one, becoming overloaded and confused, and not making the impulse decision to buy.
2) Competitor sites are gaining a greater level of trust, or meeting the customers needs and are able to close the sale.
3) People see top ads as more trustworthy and therefore are more likely to make a purchase with them.
4) Natural fluctuations in orders. Lots of orders one day, few the next.
Possible solutions:
1) Increase the bid. Will ranking higher make a difference since I'm still getting the same impressions/CTR?
2) Go for more specific terms "new blue widgets". But my competitors will still be listed for "widgets" and the problem remains.
3) Increase the number of alternative keywords no one is bidding on.
4) Improve my site to increase conversion rates.
5) Wait it out for a few more days. Maybe the competitors will drop out, and conversions will return.
Any other ideas, or solutions to the problem?
The fact that your CTR is holding steady is good, that usually doesn't happen to me. Are the other bidders not relevant to what you are selling, or is your ad copy that good?
I chalk it up to several factors, like whether it's the end of the month. For some reason the end of the month (in one of my client's industy), seems to be good. I think some people have to have to talk it over before they buy, and so on.
It's great that you are tracking your ROI. It's my feeling that it's responsible bidders like you that are able to reap the benefits and stay in for the long ride. Some others become frustrated with the results and drop out.
Good luck!
Every time I've experimented with lower rankings the click thrus and sales have fallen; and every time I've experimented with less popular phrases the click thrus have fallen slightly but the conversion to sales has plummeted.
It can be a little galling to be paying £10 per click sometimes but it works out in the long run.