Piezo signal amplifier using LM358 opamp
Aside from obvious questions as to whether your piezo is the same as whatever some random site on the internet might have  used, and whether yours is being hit as hard, there is really no way to measure the output signal with a multimeter. The pulse will be brief.
If you are seeing 1V on a multimeter it may well be railing at +3-4V (the most the op-amp can manage, assuming a +5V supply- you don't specify) briefly. Try it on the flip-flop.