I spent years with basically the same problem: the ball falling off my hand prematurely. I have had over 15 balls drilled by about four different drillers, and none of these balls ever seemed to "really fit." Then one day I got my ball drilled by someone who checked the angle of my thumb, meaning the angle at which the thumb comes across my hand. The driller mentioned that this angle was different than the average person's, and after he adjusted for it I finally had a ball that fit.
This simple little difference took years to resolve, years that I wasn't getting the most enjoyment out of my bowling, and I must admit I was rather bothered by it because these previous drillers were all "professionals" who hadn't accounted for a major fit consideration. Perhaps they weren't trained for it; I don't have an explanation. But over the years of never having a ball that fit, I had come to the mistaken conclusion that there was something wrong with the way I was bowling. The reason for this is that every fit description I'd seen or heard pointed to the fact that my spans and pitches were correct. Well, they were correct for the average person's hand, but not for mine, (which, to every appearance, looks like everyone else's hand. The difference is that if you put a clock face on my right hand, my thumb when pointed across my hand points more to 9 o'clock than the average person's).
From now on, I'll go to this particular driller, and I'd advise others to keep trying until they find a driller who actually makes their ball comfortable, because if your ball isn't comfortable on your hand, then there's something wrong and unless you have some physical oddity, the problem isn't with you.
I'd mention one thing -- if you're getting a ball drilled by someone other than your usual driller, don't give them an old ball to work off of, let them do a fitting from scratch.
So my advice would be to go to someone with the credentials and who does balls drilled by the better bowlers in your area. And keep trying until you have solved the problem.
Edit: If you live in NJ, that driller's name his name is Tony Dalfonzo, operating out of Semiz's 11th Frame at Brunswick Zone lanes. Obviously I recommend him highly.