Planetkillers Stalled

In one of my games I'm playing a Warlord, and I finally got a Planetkiller built. As you would expect, I've sent said PKer off to do its thing, but it seems that every time it blows up a planet, I can't so much as move the unit anymore afterwards. Now it's just stuck in orbit, waiting for someone to build up a force and blow it up!

This actually happened once before, and it started moving again an hour later or so, but this time it's been almost a day, and I still can't get the thing to budge.

Has anyone else had a similar problem as this? Has anyone else also noticed that PKers won't fire on a planet while in a fleet?