Bowling Ball

I’m a somewhat new bowler been playing for probably seriously around 5 months, I have a roto grip clone. I have been struggling to get my ball to spin no matter how many YouTube videos I watch or things I try. I see other bowlers be able to pick up there ball only using there thumb and that’s just not possible with my ball. Could it be that my thumb hole is drilled too big. I feel like my fingers might be coming out of my ball to early to get the full rotation I need, my ball usually comes in a bit but lands shy of the pocket. Just looking for any answers to my struggle, thanks.