****I do not know if this is true! I have not tested it! Please refute me if I am wrong!****
You can't catch more than one of the same class of monsters at one time. Here's an example of what I mean.

Say you have a thundrake in the monster bank (thunder-type dragon). In order for you to catch an icedrake (ice-type dragon) in any battle, you must first kick your thundrake out of the monster bank.
********** ---> I read this in an faq not too long ago, and I may have been mistaken. So please do not kick any monsters out of your monster bank until we're absolutely sure it's true, okay?

I wouldn't recommend kicking any monster out of your monster bank anyway until your Nu Mou Morpher has mastered its soul. It's just too hard to have to capture and recapture monsters over and over again, when the best capture percentage you can possibly get is 24% (even with Concentrate). Also, giving the monsters status ailments won't increase your chances of success. I would recommend giving them, though, as a monster that's been frogged can't give you much damage while you're trying to capture it. :P

And remember, the goblin and the thundrake are the only two monsters in the game that go extinct. Be sure to capture them and master their abilities while you have a chance! (good to keep in mind for blue mages, as well)