The ads dont show up at all - not even public service ads or blank space? If that is the case are you sure you put the adsense code in the right place in the script? There are many phpLD people on here, someone should be able to confirm where exactly it should be placed.
Maybe you could link us to the directory? I'm not sure, but if you have more than three units, don't the ads all show up as blank spaces? How many units do you have on the page?