It's very important to put in the correct amount of electrolyte before electrolysis especially if you will be reducing with any sugar type reducer like Karo, Dextrose (Glucose), Maltose, Fructose, etc.
1 ml per Liter of 1 Molar Sodium Carbonate is the correct amount.
Glucose will not work properly if the pH is below 7.0
The pH of DW is many times as low as 5.6
That is the most likely reason that you can't reduce your IS by adding glucose. The glucose will stay in it's ring form until you raise the pH up far enough to break the ring and straighten out the glucose molecule. Then, and only then, there is an available electron at one end of the chain that can be used for reducing.