Are you confident that those comments are accurate? Do you want us to assume they are correct or do you want us to explore what the real problem is (assuming there is one).
It's tough to diagnose a problem based on just those comments. I can interpret "sitting on the back leg" to mean collapsing it too much or keeping the weight back. Also, pitching tall doesn't apply to everyone - some pitchers need to start with a lowered center of gravity.
In any case, here's one thing you can try - it applies more to pitching from the stretch than the wind-up. Make sure both of your knees are bent equally. Some pitchers straighten the front leg more since it's planted further downhill. But that aligns the shoulders to level ground. Bending both knees equally will align the shoulders downhill which is appropriate since you're throwing downhill. It will also help you get the hips going as it makes you lean a bit in the downhill direction - the direction you're gonna' end up going. Just an idea.