[Randallw]

I will go through a list of things you might want to check. Please excuse me if they sound obvious.

1. Does the ship have a colony module?
2. does it have engines?
3. Are you giving orders over a homeworld?. When you order it to colonise, it must either have colonists or be orbiting a planet to automatically pick some up. Although scratch that, it can still colonise even without people (just it won't be able to build and no one will immigrate there)

I can't think of anything else, but this might be because some things are so obvious to me (A veteran) I don't think of them.

ps. oh. Are you playing a simultaneous game. In that case every empire places orders then they all run. The ship won't go off and colonise until you finish the turn and run the next.