If it says "Still growing", it's supposed to be so, it's just another outcome of the pet, there are multiple outcomes of the Valentines pups. ^^ Pets can't get unintentionally "stuck", so to say.
If it's fullgrown and still looks the same (and doesn't have PPS-tag on it) you can try hard refreshing (CTRL + F5).
Maybe this thread, "February outcomes" will also be of help. :]