I grab the widget code directly from the Adsense webpage:
https://www.google.com/adsense/adslot-add
Create the ad there and it'll save it as for you to copy and paste in the Insert Widget HTML area.
I've tried this right beside a iWeb-inserted ad and the one from Google's own HTML always shows up even when iWeb's doesn't. I don't even try to understand why, but this solution never fails me.
Also, getting the ad from Google allows more choices in the look of the ad in terms of shape, colors, and other features, so there's an added benefit.
Seth