Simple Simon.......
The most common mistake made by those starting out...
They grip the string with their finger tips pointed to the left(opposite if you are left handed)...and as they draw the bow, the draw force gets heavier, and they roll their fingers back toward them to get a better grip to handle the weight carrying the arrow away from the shelf.
Point your finger tips back at you from the start, and when the draw force get heavier, your fingers will naturally roll back slightly opening up, and hold the arrow on the shelf.
A heavier bow is no problem if you follow the same directions. Actually, if you grip the string in the 1st joints and not on the pads and follow the same directions, you can handle heavier weights much easier. You might still be over bowed, but the arrow should not leave the shelf just because you increased bow weight.
The arrow leaving the shelf comes from an improper grip on the string, not bow wieght.