Ahhh... Well, if you made one completely from 'scratch' (i.e. you did New in the Actor, Creature field), or you used an actor without sufficient information, and you are not too experienced in doing such, then that's probably your problem, there. That is, you are probably missing a lot of vital information for the Creature.
I would recommend finding a creature that belongs to a Leveled Creature (so you know its used, and therefore should be ready to go), duplicate it -- change the editor ID to what you want -- and, then make your desired changes. Also, I would suggest looking at http://geck.gamesas.com/index.php/Category:Creatures for making any changes.
Hopefully this helps, and my apologies if I had the wrong idea of your situation!