[jc173]

Is it possible you have exceeded the unit max limit? If you're playing on a large map or own a pretty good chunk of a medium map you might exceed the default limit in settings.txt. It's set for 1000, but I've had no problems exceeding that on large map just dropping mines if I have a large empire. If this is the case just edit that setting, max acceptable value is 5000. Just quit and reload and you should be able to continue.