You 'might' be putting too much pressure and in the wrong place. Find the ideal place where the box breaks over, then press ever so lightly from the front of the call working your way toward the back about midway up on the sound rail. Run the call with very short abbreviated strokes. I would also lighten, maybe you did already, the pressure on the lid. It takes very little to get nasally tree and feeding sounds on a good custom box. You might only need to move the lid 1/2" - 3/4".
Depending on the call itself, the friction and so forth...some will NOT do it. I have found Butternut for the body to work best.