This happen to me in 15w36d don't know this a feature or not (It's a bug!). I'm reload the world does not affect. But sometimes i can right clicking command blocks but not open the command blocks window (after death: MC-88168). This baby mobs bug is referenced to MC-88174.
MC-86142Click with a spawn egg on an adult mob doesn't spawn baby mobs
MC-87872Can't open command blocks after death and respawn or changing dimensions