Couple of questions:

1) How many stars does it have? Generally they don't evolve until 1.5 stars or 2.5 stars, depending on the mob. In this case I -believe- it's 1.5 stars, but I could be wrong.
2) When you feed the food, do you get the message that <Monster Name> squirms uncontrollably? If not, then it means your pet isn't far enough along to evolve yet.

Also note that it's chance; whenever you feed and get the squirm message, there's only a -chance- to get an evolution. It may not happen that day.