Shake weight beeeoootcheeees.
Work on technique, most of your power is generated there. On the bag slow your strikes down until you can perform them as close to perfect as you can get, build the muscle memory so when you do speed up the form is automatic and the power will come. This means months to years of bag work repetition, repetition, repetition.
P.S. ever have the dream where you give some dude the best shots you have in combinations and dude is straight voorhees stalking you... I hate that shit.