Page rank depends on may reasons no. of backlink/ quality backlink site content and visitors and many rasons. try to increase backlinks of site. it will boost your Pr. For more info. u can read this blog. http://seo-secret-blog.blogspot.com
not only back link is responsabile for good PR but how much visitor coming on your site it also matter for your site PR.
the only way it seems to overcome the sandbox effect is to keep doing link building or to get some links from very high authority (PR) sites.
Hi, I have just read through the whole thread .... Now my advice is (if nobody has already said it ) get more backlinks from same themed sites and from pages with HIGH PR PAGE LINK that are DO FOLLOW! Enjoy
If I have understood it correctly page rank is only determined by backlinks. Unless that has changed. So in that case you must have lost some of your links.