Insulin resistance (IR) is a condition in which the cells of the body become resistant to the effects of insulin, that is, the normal response to a given amount of insulin is reduced. As a result, higher levels of insulin are needed in order for insulin to have its effects. The resistance is seen with both the body's own insulin (endogenous) and if insulin is given through injection (exogenous).
One of the actions of insulin is to cause the cells of the body, particularly the muscle and fat cells, to remove and use glucose from the blood. This is one way in which insulin controls the level of glucose in blood. Insulin has this effect on the cells by binding to insulin receptors on the surface of the cells. You can think of it as insulin "knocking" on the doors of muscle and fat cells. The cells hear the knock, open up, and let glucose in to be used by the cell. With insulin resistance, the muscles don't hear the knock as well (they are resistant), and the pancreas is notified that it needs to make more insulin, which increases the level of insulin in the blood and causes a louder knock.
The resistance of the cells continues to increase over time. As long as the pancreas is able to produce enough insulin to overcome this resistance, blood glucose levels remain normal. When the pancreas can no longer produce enough insulin, the blood glucose levels begin to rise, initially after meals when glucose levels are at their highest and more insulin is needed, but eventually in the fasting state too. At this point, type 2 diabetes is present.
Besides the insulin, something you can try adding is...if Charlie will still play...get him up and moving around a bit. Or even rolling around on the floor if needed. Tilie would play chase anything anymore, but she would happily roll back and forth and swat skyward at things. Exercise can be one type of therapy to unlock the doors of the muscles.
In the meantime, it may be time to really discuss or consult with your vet or ...I believe you said the vet across the street who was more amendable to home testing. It may be time for that insulin change. Especially since Charlie is redeveloping signs of diabetes (tiredness).