WPR is a pure mathematical number based on the sharing of PR values between all
the sites you link to and who link to you. That's all it is. Just a damn
number.
Now what Google tries to do with it's SERPs is return the most relevant information or site. How can they judge relevance from a number? The answer is that the cannot.
So in addition to the PR calculated from a link their algorithms also calculate a relevance value for the link. It is mainly this relevance value that determines the place of the site in the SERP.
How do they calculate this relevance value. Well as far as we can tell they calculate it today from the anchor text used with each link. Maybe in the future they will use other techniques like the theme of the page that the link comes from, but today it appears that the main driver is anchor text.
Now as to your site dropping after it got a PR rating. There may be two possible explanations. The first is that Google always gives a new page extra attention or emphasis. If they have not yet updated the links I think they may even place all their evaluation on the on-page factors like title, heading and content. So if you page is reasonably optimized for these factors it will show up well in the SERPs while Google figures out the linking factors.
Once Google has checked and evaluated all the links them the page becomes part of the normal population and has to compete on that basis. You say you have 24 backlinks. That is not many. How many links do the sites that rank in the top ten have? Also what anchor text are your links using. Does the anchor text contain your prime keywords?
So get more links. Don't worry about PR. Just make sure that you are using the right anchortext.
