I have some sites where i put AdSense on it and only ONE 120x160 unit shows up. I put a second one on the site, maybe a 250x250 or 468x60 with the other unit, and it just wont display! It just comes up blank. I dont understand it. You can have THREE ad-units + link units + search bar....so its beyond me why on some sites i can only display ONE unit?! G.
it must be something to do with your site layout. if your site is using tables or if you are using blog trying to add the code in the pre-made template, then the ad size you are selecting must not be fitting into the space. what you can do is select a smaller ad size and see if it shows. if the small ad unit shows there, then you know its template issue