With one ad, I'm presuming that you mean only one ad unit displays ads. This is due to a couple of things: a) There's not enough advertisers for the topic b) Your pages don't have enough content c) Your pages are relatively new and Adsense (Mediabot) hasn't thoroughly spidered it yet.
I used to have a skyscraper + links unit. Yesterday I've changed the layout a bit and changed the ad units to skyscraper + leader + link unit. Any idea why the skyscraper stopped appearing? Doesn't even show the public ads It's actually just one page with a FAQ. Any help would be appreciated. link
The most likely cause is that there isn't enough content on the pages to support all the ads. I'm taking a guess here, but in all likelyhood the skycraper is lower down in your HTML code. Google will display ads for the first ad unit in your HTML code, and if there aren't any ads available for the other ad unit it will simply not display or display a PSA.
In my code it actually goes leader then skyscraper then link units. But, I've disabled some ads to see the effect and if I disable the leader and link units the skyscraper shows a PSA (well, at least it shows anything). Tried it with the link units and it disappeared again. I guess no skyscraper ad is planed to be displayed on my page that's why it's not showing up. Maybe if the topic gets much more popular it'll come back. Thanks Crusader