One of my sites were ranked on page 5 in google for one of its main keywords. I wanted to improve this rank, so I bought a hardlink (with the anchor that is the same as the keyword) from a very strong pr6 site. So what is the result? Well, the rankings has dropped steady since I gained that link. page 6, page 7, page 10... now about 2 weeks later the site now end up on page 14 in google! I don't get it... do you?
lots of factors to consider.. first just because the site says it's PR6 does not mean it's a real PR6, second if it is a PR6 site that does not mean it's a 'very strong' site as PR can be bought anyway. Other factors, 1 link will not do everything, on page / off page factors, lack of content, lack of good content, to many ads.. the list can go on and on
Maybe it's just a coincidence .... maybe you should be on 14. Did you build other backlinks during the 2 weeks period ?
If you are able to get backlink from those page rank 6 website that definitely you should have moved ahead rather than going backward in terms of SERP. But here even the theme of the linked website plays a huge role. Further also consider whether those links are do follow or no follow.