Keep feeding it correctly, or try another item for food. But when the monster is ready to "evolve", it gives you the reply somewhat akin to "the lamb is shaking/quivering with excitement" or some word pertaining to him shivering or shaking. THAT'S when it is ready to evolve by leaving Mog garden then re-entering. Also make sure you are doing the correct slap/pet/yell/poke interaction or it may not evolve any where as fast, or even not at all until that has been done a few times.