Arduino Midi Marimba

I have a Marimba and would like to Midify it. The plan is to put a piezo disc underneath each bar and connect the piezo and the bar via foam pads. The signal goes into an adruino with a mux shield. I am currently in a test phase only using one bar. I use some electonics to condition the signal from the piezo (http://leucos.lstilde.org/wp/2009/06/piezo-transducer-signal-conditioning/). I have a simple sketch for the arduino. The bar is identfied with a number "49" (1Byte). The analog signal from pin zero is devided by 4 to reduce it to 1 Byte. The max patch searches for the peak value and triggers the midi note.
It works fine with soft hits but not with hard hits. In fact I seem to get quieter responses from hard hits although none of the midi velocity values are anywhere near 127. Any comments greatly appreciated.