SacII exhibits strong site preference. SacII requires two recognition sites to activate cleavage. Try reducing the reaction volume to 10 times the volume of enzyme added, or adding 30 units/microgram and incubating for 2-3 hours. Adding an oligonucleotide which contains the sequence will help if there is only a single site on the target DNA. (Conrad, M., and Topal, M.D. PNAS USA 86, 9707-9711 (1989), and Pein, C.-D., et al (1991) NAR 19; 5139-5142).