In the windup I can get good velocity with seperating my hips, but using I slide step I’m at a loss at how to throw harder.
As far as I can see your difference is that your hand break needs to be earlier on the slide step vs your regular post. As soon as you move your foot forward, start to break your hands quicker and with more purpose. Give it a try.
With a slide step, it’s more difficult to sequence the elements of your delivery properly and with good timing. For example, a lot of you pitchers will overlap hip rotation and shoulder rotation which reduces hip and shoulder separation.
The trick is to slow down the lower half without looking like your entire delivery is slow (which would encourage your opponent to run). Try taking your front knee back towards 2B a little bit and get the hips moving forward while you do. This will buy time for the shoulders to stay closed longer while the hips open up. The result should be more hip and shoulder separation and better velocity.