I have the Vinmetrica 300 & believe its results are accurate. Is your sulfite source fresh? I find that using the plastic syringe is difficult to precisely measure the amount of titrant used. It's also difficult to dispense one drop at a time, which gets critical as you reach the end point. A burette would probably make that measurement more accurate.
I have not used their customer service but I hear it is excellent. They may suggest you test yours against a solution of known SO2 level.
NS