An inportant thing to remember is that just because you are right-handed does not mean you are orthodox, and just because to are left-handed means you are a southpaw. Look at Oscar De La Hoya, a left-hander who fights using an orthodox stance.
Uppercutting from the lead is a tricky punch. You have you bear in mind that when throwing from your "power" hand, you hips will have to move more, and the more movement you have, the more power you generate.
To demonstrate the point, try this. Stand in your normal stance (which for you, I assume is southpaw) and stick you right hand out in front of you as far as you can. Hold it there. Then, stick your left hand out as far as you can. You will see straight away that the right hand reaches further than the left. When punching, you will have to move your body to get your left hand to reach the same point as your right. The same principle applies to your uppercut.
Try the exact same excercise as above, only this time uppercut. Hold your right uppercut, then do the left. You will see that your right is once more, ahead of the left. If you need your left to uppercut the same target as the right, then you need to move your body (by twisting the hips) to get it to the same place.
So it stands to reason that your lead uppercut will have less momentum usually that your "power" hand uppercut. That is why a lot of people throw the uppercut with their lead hand.
For practice, don't use a heavy bag for uppercuts. Most people only half-complete the punch, as the bag stops them following threw.
Use hook-jab pads, with the pad turned downward, so that the pad-side faces the ground.
Saving that, find a light bag and hoist it up to around 180 cm (about 6 ft or so, depending on your height!) and uppercut the bottom of the bag. Do nothing but lead uppercuts while practicing at first, then move into combos, then combine them with footwork/body shifitng.
As always, I may not have explained that all very well, so any other questions PM me or post back.
"Let your food be your medicine, and your medicine be your food"