NeilF,
I too have noticed on a few occasions that metadata can't be found when importing, and have assumed that the metadata service (presumably somehow linked via the Sooloos servers) is down for whatever reason. As you've found, if you leave it for a couple of hours, it's usually back up and running.
A bit annoying at the time (and causes a bit of head scratching the first time it happens - as you say, you start trying other CDs and wonder if your system is at fault), so it pays to have something else to do instead and leaving the importing for later!
Ian