Have you ever practiced your ball toss separately trying to throw the ball up while the rest of your body is totally stiff?
I remember the classic drill that a number of coaches do:
I still see a lot of this happening and the problem is the toss should be integrated with the coiling/turning of the body. If constant static tossing keeps happening it delays the process to incorporate the feeling of one flowing serve motion with the toss and be consistent.
The actual serve is started with the turning of the lower body and as you turn to the back and orientate your body skyward the tossing arm will naturally go up as one.
Also the tossing arm path does not go straight up and down as that does not resemble a flowing motion that works wth the bodies coils.
A friend of mine does a great serving course you can learn online to help you understand better when your not able to be on the court with your coach. Check it out here - SERVE UNLOCKED