I have recently bought a few valid PR domains and transferred them to my account. I enabled whoisguard on the domains to make sure that the whois data is private. A few days after, about 50% of the domains (with good backlinks) went from PR4 and PR3 to PR0. The other 50% remained unchanged in regards to PR. Why is this happening? How can I prevent Google from changing the PR from these transferred domain names?