Mezzy
I see your HBA1C is 6 so that means you rarely run high. If you were running high, Id say that was the reason youre not putting on weight.
To build muscle, you need to be eating protein. Carbs are more for energy. Are you drinking a good protein shake? Basically you should be consuming 2 grams or protein for every killogram of your body weight per day if you want to build muscle. You need to divide that into 6 portions a day. Brocolli, sweet potatoe, green beans, beans, cabbage are all great.
I always had issues with weight. Soon as I made sure that my Bloods did not go over 11, I gained 6 killos within 4 months. The minute your bloods go over 11, you start peeing more - basically peeing out all the proteins etc that your putting in. If youre peeing too much....you wont put on weight. If youre not, then youre not eating enough protein.
You should also research your workout. If you doing cardio or high rep, that may be another reason. Low rep high weight with a decent recovery time will help put on muscle. 3 x good sessions a week are good enough.
Hope that helps