They probably need the comb built up in order to see through the sights when their cheek is rested. You can build it up with anything you have handy, such as foam, bandages, cardboard, tape, etc., or you can get a stock with an adjustable comb.
The way you know if you have the right comb height is when they can get in position, get their sight alignment, close their eyes, breathe, relax, and open their eyes, and still have their sight alignment.