cant spawn zombies with spawn egg!!!!!!!!!!!!!!

started a world in 1.7.2 in creative mode put zombie spawn eggs in my inventory and reloaded the same world in 1.7.9 the next hour and when i tried to spawn a zombie it would spawn and despawn within 1 second and growl once i dug down into a cave and tried to spawn it and it didn't work either and after that i re launched it in minecraft 14w21b