It's simple:
* If you want to send new messages and want them private, encrypt them. It's clearly written on the PM page that this is the solution.
* If you want old PMs become more private... well, I don't think that theymos will help you because he doesn't like censoring and also doesn't like to create dangerous precedents.

And since this is common knowledge, I agree with Loyce: most probably your request PM was simply ignored.