It is probably that you need to shift your weight (pivot) better.....
The left in this combo is different than the standard jab - for the standard jabs you shift your weight forward onto the front foot. For this, you are pivoting your weight backwards from the front foot to the back and using that body shift to pivot your shoulders and hips to force the left out there.
Try this walkthrough: After your right is out and extended, you have your back foot on the ball, most of your weight is over the front (left) foot. Your left should be up guarding your chin, and chin tucked.
Now, draw the bow - pull back hard with your right shoulder and at the same time, shift your weight back and thrust your left shoulder forward propelling your left fist out (your hips should also pivot simultaneously). You should end up with weight over the back foot, right hand in guard over the chin and left extended straight out. Hold that position and it will look similar to how you might be ready to shoot an arrow, except your right elbow should be tucked and not flared in true bow shooting form.
This is an extremely useful combo - its very unorthodox in its rhythm, and thus has a high connect %. I believe its also what Buster Douglas used to finish Tyson (1-2-1 as I recall) and I've also seen some other pro boxers using it to good effect in the ring.
It does take time to get comfortable with though - spend a few minutes every session on it, and in time you'll find it will feel comfortable and you can in fact get some good pop on it. It will never have the power of a rear straight, but it trades off less power for higher % of connecting.