The previous comment stole much of my material

A couple things, though..... If you are reaching with your jab, try pushing off your rear foot instead of stepping with your front foot. You won't reach and it will do wonders to get you inside. The key to landing the left hook or uppercut after slipping inside the jab is all in the step you take. The step should be small- this is a quick move- and you have to bring your right foot up instantly as well or your balance will be off. To get your straight right working in the ring as well as on the bag.....practice your power punches with the bag coming towards you, as you want your opponent to be doing in the ring when you nail him. While sparring, try slipping inside the jab and landing the straight right to the heart area. Getting this timing right should also help with the slip/hook move. Once you can land the right to the heart, practice slipping and throwing it to the chin, UNDER his jab.

If you've got the catch-pivot-jab counter working well, you can get it all. Good Luck