To the thread starter, the primary reason why you are no longer taking the No. 1 position could be the fact that you are facing some kind of competition for the term.
Apart from having on page search engine optimization elements to compete, you need to start building links that have an anchor text bearing the keywords that you are targeting. For example, if you want to target "music label shops in new york", you need to build links that point back to your site using "music label shops in new york".
PR will hold no values if you are not building your links as the method that I have specified above. But when facing tough competition,
PR will actually come into play.