What band sizes are you expecting from your 170bp pcr product. Did you cut the pcr product in pcr buffer or was it purified on a column before cutting. Does the 170bp contain any other NIaIII cutting site. When you say it did not work do you mean that the 170 base product did not cut or that it did cut and has disappeared but that you cannot see the cut fragments on your gel?

170 bp is a small band. You need a a lot of copies to see it on a gel. Start with more PCR product.