The cardinal rule of using the MDRS is to not attempt to remove dents from a tuba or sousaphone while in the instrument is in a vertical position (standing on the bell.) Always lay instruments in a horizontal position during dent removal operations. If the ball should come loose from the tool and fall the wrong way into the tapered tubing, it can become irrevocably lodged. If this happens to you, try the following options:
Tap on the ball through the brass to dislodge it. Don't give up on this method right away - it works a lot of the time.
If option 1 does not work and you have 2 people in the shop, try using 2 of your MDRS tools by putting one tool on each side of the stuck ball to the best of your ability and with sheer brute force, pull the ball back up the tubing enough to break it loose.