You're simply too stiff. It looks like you're sawing wood. Relaaaax.
Some will say you're dropping your elbow. Some might say you're actually raising your elbow on your back-stroke (0:27). That's all fine and dandy, but the primary reason you're probably not excelling is due to the following:
1) First and foremost, you didn't find center ball on any of your hits. That alone will make you miss more than anything having to do with elbow positions. Even if you drop your elbow, raise your elbow, whatever--- if you're perfectly on the vertical axis, you should at least make the ball or come close. You're erring to the right side of the CB on each shot. You're probably over-cutting shots to the right and undercutting shots to the left. Left cuts are probably higher % than your right cuts (just a guess).
2) You're not smooth (too stiff)
3) Your stance looks really uncomfortable (which contributes to you not being smooth)
Dave