On one of my web sites - http://whatjapanthinks.com - you should see that right in the middle of the page is a browser error about googleads.g.doubleclick.net being an invalid address. Why is this? Other web sites of mine that use similar-sized ads work OK, and the skyscraper on the left is also OK. I've not changed anything as far as I am aware, but I've been getting these errors on and off for a week and they are really denting my income!
Are you sure? Where are you located? AdSense stats say about 1,300 displays of my header links, but just 10 on my 300x250 block.
Ack, fixed it - my fault - I'd updated the theme, which had been created on a Mac with CR line endings and was uploaded that way, which meant the JavaScript wasn't being output correctly.