Why is my Straghtner not working like it use to?

It could be because of that, but it could also just be your hair. Make sure you use a heat protectant spray/serum. Your hair could be super dried out which is why it looks frizzy. I went through this a while ago, so I stopped straightening my hair everyday. I guess it just eventually gets to a point when your hair becomes damaged and the straightner will only dry it out even more.