What is the role of the capacitor within the assembly
You are low-pass filtering the piezo-sensor's voltage by adding a capacitor.
Since your fluctuations (pressure applied) most probably lies in the high region, low-pass filtering has gotten rid of it.


  the piezo generated a positive pulse that I wished to get rid of


Are you sure that you want to get rid of +ve pulse at the input to the opamp?
You need to reverse the polarity of the diode then.