Well, I am going to explain the forces which may cause for an errant toss. 1) shoulder rotation 2) balance and 3) alignment. Here I am going to call #1 the "merry go round" and #2 the "seesaw". And #3, the 2 to 7 LINE.
Here goes #1: This is what I call the "merry go round effect". You're supposed to be nice and look over at the opponent before you serve, right? But where are your hands? Where is your body weight balance centered? Is it mostly at your chest? I would tend to think when looking at the opponent, the weight is at 12-1 o'clock, if I had a clock dial on the ground.
So what's the "merry go round effect"? Well, as you're done looking at your opponent and begin your motion, you move your weight and balance towards two o'clock. Your toss arm goes up at two o'clock and your hitting arm goes winding around at 7 o'clock at trophy. You've done this with shoulder rotation, your left and right shoulders revolve like a merry go round, correct?
But here's the thing. There is also what I call the "see saw effect" #2. Did you lean forward into the court when you began all this motion, like Federer? Or did you hunch over with "gorilla arms in a bear hug" like Sampras with the front toe up? Did you start neutral and level with the front shoulder and rear shoulder, like a nice mild mannered person? Or did you do that abbreviated Jay Berger motion with the toss hand high and the racquet back and rear shoulder down, like a sprinkler head ready to bust? lol
So every service motion deals with the "see saw effect" and the "merry go round effect". And it is very easy for everything to be inconsistent. Unless you sort everything out, right? Let's go sort it out. :)
Well here is how I sort it out. If I drew a line from 2 o'clock to 7 o'clock on the ground, using the clock dial on the ground analogy, I have a diagonal LINE from where my toss hand is and to my hitting hand is in trophy. Imagine that line. Really imagine it. Or if you have to, draw a line in chalk on the court from where your toss hand extends (point 1) and where your racquet extends behind you (point 2). DRAW the line.
I want to turn my merry go round shoulders, the lead shoulder from 11 to 1 o'clock, and extend my toss at 2 o'clock. If I want to use the Sampras method, I do the "preying mantis" both arms bent, and then stop the shoulder at 1 and extend the arm to 2. I do not want my toss hand to go beyond 2:30. If I were at 3 o'clock, the ball will arc over my head, LEFT to RIGHT. If I were at 1 o'clock the ball will arc back into my face, FRONT to BACK. Not much fun there. lol So I am lifting the toss at 2 o'clock, using the seesaw effect at this point, tried and true. I can hit flat, slice there. At 2:30, I can hit topspin and kick.
So in essence, you want the toss to go ARC BACK into your swing path, along the 2 to 7 LINE for flat and slice. For the topspin and kick you want a little more LEFT to RIGHT in the toss, 2:30 to 7:30, so you can get the back side of the ball, and make contact from 7 to 1 on the ball if it had a clockface on the back of the ball. If you got a flexible back and shoulder blades, try 3:00 to 8:30 like Sampras. I'm not flexible like Gumby and most people aren't. lol
So we're done with toss placement. I COUNTERBALANCE this toss with "weight" on my right foot. So imagine balance is being distributed, the hand is slightly to the right of the 2 to 7 imaginary LINE, the right foot is balanced to the left of the imaginary LINE. I may actuate the balance of the right foot with a slight bend in the right leg. If I don't bend the right leg, I put the weight on the inside of my right toe. In any case, I am balanced around this diagonal 2 to 7 LINE. Toss hand to the right of the LINE, right foot behind the LINE. Or think of this: your left limb (arm) is up, your right limb (leg) is down. BALANCE is there!
If I don't want to deal with this merry go rounding business, I just start aligned around the LINE. I begin the toss at two o'clock and use a seesaw mostly. I start with my hand low over my front pant leg and lean over like Federer into the court and "see saw" up. Here, I don't think about my right foot to counter balance, I have my head and spine to counterbalance. I lean my head to the left as I am tossing and straighten my spine. My right foot comes down, but my right leg does not flex, as Federer does it. He is evenly balanced on his legs and go straight down after the toss and platform squats.
I call this the "water pump method". lol When I start, I lean over with my toss hand low and my right arm straight and up like a hatchet behind me along the LINE. I don't look at my opponent, but bounce the ball. Don't I look like a water pump? Then I "pump" or "seesaw" the ball, tossing up.
If you look at pictures of Sampras and Federer or Raonic, they lean their heads to the LEFT to 11 o'clock as they toss. I believe this straightens the spine and keeps the balance of toss along the 2 to 7 LINE. It also opens up the hitting arm to arc up and strike.
Balance, balance, balance. Rotation, see saw, and alignment. See if this explanation fixes your toss. Why don't instructors explain all this? Could you imagine explaining all this on the court? Maybe they don't know this stuff! Well, the toss revolves around these issues. Take the lowdown and run with it. You got it free from me!