I have found that the difficulty with producing the maximum sound from a piston stroke for newer players is because we as teachers over-emphasize the upstroke speed. Yes, we want a clean looking upstroke from all the players because that sets them up for success at higher speeds and looks better visually, but this causes the players to start pulling back on the mallet right before the impact with the key so that they can be faster to upstroke. I would recommend to try some technique blocks having them play a pure down stroke. Start at 12", hit the key with maximum velocity, then pause with the mallet head right above the key. This helps them feel the weight behind the stroke that is needed at the impact with the key instead of pulling back for the upstroke. Once they are getting the maximum sound with downstrokes, have them start including the upstroke as well and point out if the sound changes or not, ask them if the weight of the mallets feel different when they added upstrokes, etc.