Make sure you are not trying to spawn an egg in a worldguard region with the flag "mob-spawning" on deny. Also make sure you have permissions to 'build' in that area and permissions to use the eggs. Is your game mode on peaceful? This should be anything higher than peaceful. Lastly, make sure you are not trying to spawn mobs on places where they don't belong. A squid on land for example. Some mobs also won't spawn in too confined areas.