Assuming your span/fit is correct, you may need to use tape in your thumb hole to tighten up the fit.
some people use white tape in the front or back, or both front and back side of the thumb hole.
I use magic carpet tape in the back side and white tape in the pad side (front) of my thumb hole. this helps me stay in the ball longer while having a relaxed grip. But, I also have a fit that allows me to have a relaxed grip.