There's a few things You might consider. First, were they dropped domains? Drops domains have a higher chance of incurring falling PR than non dropped domains. Second, do you still see backlinks wiht PR value when you do a 'link:' command on google? If google doesn't show any backlinks form the link command, there is a change you may lose pr value in the next couple of pr updates. Third, and most important, add new backlinks. buying domains with pr value give you a nice little jumpstart on your backlink gathering. But it shouldn't REPLACE your backlink gathering campaign. You should be able to gather enough good backlinks to ensure your new domains retain their pr value by the next pr update.
I totally agree with the above... focus on the SERP's than the PR value of your site unless you are looking to sell some links ;-)
There is nothing you can do now that will effect this update (any day now). This update is a snapshot of your site's backlinks as they were a few months ago. For future updates, do as other have suggested, build links to your site. But links should be from related pages or else PR may not pass. /*tom*/