as has been said, your meant to hit the bag with the 1 and 2 knuckles, with straight punches aiming to land dead center.

it might be the angle of the punches your delivering. practice the "wideness" of your hooks and uppercuts slowls, each time twisting at the end and making sure that they land on the first 2 knuckles ech time. keep the wrist straight when you punch, otherwise you stand to do yourself alot of problems, and whip the punch through with your body rather than with your arm