Just go back to the basics.......serving is a throwing action!
Next time you step on court....stand on the baseline and throw a ball as far as you can without a run up. Do this several times....relaxed but rapid throwing action!
The majority of people will naturally stand to the side.....rotate the shoulders and hips then throw around and out and WHAM...there it goes!!
Now add a racket.....and reach up for the ball....you need a continental grip (your coach should have shown you this!), then just concentrate on getting the ball over the net.....with your relaxed `throwing action`.
The amount of players I`ve worked with that as soon as they develop some consistency, their next question is...`how to make it faster?`
NO NO NO NO NO
Once your getting the hang of it.....work on direction! Can you get the serve where you want it? Consistently?!? Whats the point in power if your hitting it to the wrong place?
Obviously it would be alot easier to help if I could see your swing but the above gives you a solid foundation to work from.
Sorry for the long post but I am very passionate about tennis so am keen to help!
P.S. As said before.....the ball toss is very important......you could have the best serve in the world, but if you toss the ball in the wrong place then I will never get to see it!