Problem with spawn egg

I made a mob and tried using its spawn egg, but the mob doesn't spawn. It is a creature type mob so it SHOULD spawn on any difficulty but it doesn't. I even tried using /summon and the mob still isn't there. It says that it summoned the mob but there still is no mob. Other than its mob type, its setting are just the default settings. I'm not too sure why this is happening, because it shouldn't be.