Yes, as LeeD says you've got to explode up from your legs more.
It looks like you are getting a reasonable knee bend, just not exploding up.
But you are much improved on "dropping your left shoulder".
Getting better at your serve will take time, but you are on the right track.
You still are bringing your right leg forward, although not as dramatically swinging it out to the right [thanks for SA for the correction of legs
] now that you are "dopping the left shoulder" more.
You may want to try just "shadowing" some serves without tossing a ball to get the feel of launching yourself up and landing on your LEFT leg with you RIGHT LEG KICKING BACK.
Leg Kick on Tennis Serve http://blip.tv/fuzzy-yellow-balls/le...-serve-1190196
I really think if you could just feel the power of launching yourself straight up, and landing this way by shadowing swings without a ball will form some "muscle memory", making it easier to incorporate into your serve.
(And you will fell great just having that sensation of really launching yourself up.)
Perhaps you don't understand how to toss more into the court.
Check out this overhead view of the Sampras serve toss:
For a first serve toss you have to start with your hand/ball close to your back foot (pic one) and end up with your hand/ball near your front foot at ball release (pic 4) arm.
I warn you, however, that this forward sweep of the tossing arm is counter to the backward lean with your upper body as you need to form the bow shape.
It takes real concentration on bringing the arm forward not to let your upper body lean forward into the court, or else you will end up with a "reverse" bow shape.
So notice that Pete doesn't allow himself to lean forward at all, even as he brings his arm forward.
And note also that it is AFTER ball release in pics 7-10 that he is getting that really big movement of the left hip into the court as I earlier advocated, and xFullCourtTenniSx did in his recent post.
That big lean back with upper body/forward hip out really helps launch yourself "up the mountain" when you reverse the bow shape even more violently.