Have you tried the door frame drill? It will demonstrate where, for your body, is the most powerful position, and which muscles give you power.
Since some people learn better by reading: Stand in a door frame sideways. Situate your feet so that you just took the last step in your X-step and the disc is still behind you. The door frame should be at or behind your trailing (left) foot. Grab the door frame where your disc would be at full backswing (you may need to adjust your feet to get the door frame in the correct position):
Pull the door frame like you are trying to rip it out of the wall, toward your target. You might first feel your shoulders or triceps, but there will be no power. Pull again, maybe adjust your hand height in the frame slightly. You might feel it in your side. Pull again until you are aware that the strongest position is when leaning into your hips, shoulder forward, where you might feel like you are about to sit in a chair that is just out of reach of your right butt cheek. That's where the power is. From your butt through your back. That's the correct position for your body to throw the disc. Some people do not 'like it' or decide not to throw that way, but that's the position your power and mechanics put you.