More than likely its just genes and your own personal body. We’re all individuals and as such, different things happen differently for some. Take the example of liquor. Some people can drink their weight in the stuff while others can only handle a few. I’m a small fellow but ive got quite a but of Irish in me and luckly I can drink quite a bit even though by theory I shoulnt be able to because of the smaller size. That’s just one example. Drugs and medication work the same way. My theory is somehow your body’s just built an immunity to the novicane.