Most Royal Canin is very high in carbohydrates, too much so for a diabetic cat. Your vet probably isn't aware of this; they get very little training on feline nutrition in veterinary school, and when salesmen come to the practice selling food that's "perfect for diabetic cats", the vets say, sure, great, of course we'll stock it! Where are you located? Since your kitty was started on Caninsulin, I am figuring you are not in the US. If we know where you are we can help you with links to low carb canned food that is readily available in your part of the world, that won't cost an arm and a leg!
Caninsulin is one of the harsher, fast-acting insulins. It has two components that work to bring down blood glucose; the first one starts to kick in early in the cycle, and can cause very sudden, steep drops in BG, while the second one works later to affect a slower, smaller drop. Many feel that they don't get a full 12-hour duration from Caninsulin. I used Vetsulin, the US equivalent, for many months; my cat did reasonably well on it, and I did not experience any problems with duration. However, I have recently changed to Lantus and my kitty and I are much happier with it.
Without having more data it is impossible to say why your cat may still have high BG, but I feel pretty certain the Royal Canin may be at least one of the culprits. If you transition your kitty to a low carb diet it needs to be done gradually, mixing portions until kitty has been completely switched to the new food.
One very important caveat: you MUST be home testing and in a position to closely monitor your cat during and just after the transition! A cat's BG can drop as many as 200 pts due to this type of diet change. This can result in hypoglycemia, a very serious and potentially deadly condition.
You can learn more from the following links:
http://www.felinediabetes.com/FDMB/threads/transitioning-your-cat-from-dry-to-wet-food.956/
http://www.felinediabetes.com/FDMB/threads/how-to-treat-hypos-they-can-kill-print-this-out.15887/
http://www.felinediabetes.com/FDMB/threads/jojo-and-bunnys-hypo-tool-box.2354/
Kudos to you for mastering home testing so quickly! You are doing a great job!

