My Lady has been diabetic for almost six years now.
I can tell you why the tablets aren't working. Unlike human diabetics who can often control their blood glucose by diet and oral medications, dogs are always Type I diabetics and require twice daily insulin injections.
I'm sure Zack's recent episode when he was sick was ketoacidosis which is caused by not enough insulin. Ketoacidosis is very serious and can be life threatening. I'm sure that's why he is having seizures.
Diabetic ketoacidosis is caused by a lack of insulin or an insufficient amount of insulin. Since the lack of insulin means that glucose in not able to be used, the body searches for a new source of energy. In this condition, the diabetic breaks down body fat (lipolysis) to use as energy. During lipolysis, waste products called ketones are produced. Ketones are eliminated in the urine and through the lungs. Under normal conditions, the body can tolerate and eliminate ketones. But in diabetic ketoacidosis, fats are being broken down at such a high rate that the body can not eliminate the ketones fast enough and they build up in the blood. In high amounts, ketones are toxic to the body. They cause the acid-base balance to change and serious electrolyte and fluid imbalances result.
You need to find a vet asap who understands canine diabetes. Untreated diabetes leads to organ failure and death. Eyesight is one of the first organs to fail. I'm guessing Zack may already be blind.
This is a wonderful website that is full of information about canine diabetes. There is also a forum that is a great source of information and support. Please feel free to pm me if you have any questions.
http://www.petdiabetes.org/contents.htm