I noticed right away that your thumb moves a whole lot more than mine does. Does it do the same on your right hand? Also have you tried watching your right hand and then copying that, at a slower tempo maybe, with your left hand, making the exact same motions and all? That's what I had to do to iron out my finger technique with my left.
You can see Dave Weckl do it here http://youtu.be/tEuJiapUoK0?t=29m43s
My other thought is it's just that your left hand is either lacks the strength of your right, or the coordination. But either of those should have gotten better over the months you've been practicing, unless you've been practicing wrong, which does happen.
Grab your pad and play in front of a mirror, or video tape both of your hands at the same time and do your finger technique and watch...what's different between the two?