Why am I still getting new and painful acne after taking accutane?

I’ve been on accutane (40mg) for three months, first month i took one pill every other day, second month one every day, and now at the end of the third month it’s one pill every other day and two on the days I don’t take just one. I take it with food, and I’m still getting new and painful acne on my chin. Nowhere else. I’ve tried everything, changing my pillowcase, washing my sheets every week and still nothing. I’m just frustrated, no acne treatment has ever worked and I was hoping this would.