Try obvious things 1st. Fluid level-OK. Kick down adjustment-OK. Band adjustment can cause problem. As mentioned, pressure test. Check it at all ports & compare results with service manual. If OK there pull the pan & look for anything out of place. Band anchor strap, servo snap rings, band levers, loose valve body bolts/ screws. If you pull the valve body down check the servos for leaks with compressed air by blowing in through the pressure check port & looking for leaks(bubbles or hissing) around the seal rings. After that you probably gotta pull it back out & start tearing it back apart.
I hope this helps.
