Why wont the ick (white spots) fall off my fish?

Alrighty, I have a fish that I bought from the pet store the other day and it was infected with ich. (Ichthyophthirius multifilis) I bought treatment and the directions say to add treatment, wait 48 hours, add more treatment, and then change 25% of the water. BUT I have done this and the ich will not fall off of my fish. Should I just repeat the treatment?