"A couple of hours" is insufficient time. You need to give it several days at least. YPN is slower than Adsense at "settling in." Patience is required.
Also, if contextual targeting is not good, you can override it using so-called Targeted URLs, where you can specify ad categories for up to 50 pages or directories of pages. So, for example, you specify to only show consumer electronics ads on a page or set of pages. This is a very useful option. (In theory. In actual practice it sometimes doesn't work too well, mainly because YPN has limited numbers of advertisers in some categories.)
So, if you also do Adsense, here are your choices:
--Adsense contextual (generally good targeting)
--YPN contextual (often poor targeting, but give it time)
--YPN categorical (generally good to okay targeting, but not as pinpointed as Adsense)
You have the best of three worlds.