Are you waiting 48 hrs after you stabilize before adding sweetener? I think it may be 24 hrs after but I wait 48 hrs, check sg then add sorbate check sg again before you add sweetener never had a problem. Another problem might be.. if you purchased the sorbate at hbs it might be expired. Your hbs probably don't sell it fast enough. Your best bet is to order it or pick it up at a winemaking store that gets a lot of traffic hope I helped!