Whenever I get a new leather holster, I always put my guns in one or two sandwich bags and put them in the holster to stretch the leather a bit and let it sit over night. Then I draw and reholster the gun multiple times until the baggies are worn out, and then put my firearm itself in the holster to let it sit for a bit (few hours). After that, I don't have any problems. I don't use a thumb snap, but should work the same for you to stretch the snap part. If you want a snug fit, it's going to be a little tight at first. Work it in, then it will fit like a glove.