How old is the site?
I'm starting to think there's a negative age factor (or perhaps an average of age and incoming links over time) in toolbar PR.
Jenni.me went up to a PR2 (from 1), yet my partner's site - which has only been live for a couple of months and has many of the same (but fewer) links - has gone from unranked to 4. Our domain name extensions are different and so are our hosts.
And yes I am annoyed 
My PR6 site has gone down to a 5 as well. Everything else is up though.