Thanks all! Problem solved. I tried to connect all the resistors to the ground and it did work!
I'm having a problem regarding a circuit on a perfboard. I try to use five piezo sensors which send MIDI messages to my Max Patch. I first did the setup on the breadboard and it worked well. Then I tried to move all I did to a perfboard, but it didn't work (see the pics).
I understand the perfboard doesn't have the connections between each point so I connected using solder/jumper wires. I can see there are a bunch of MIDI messages running automatically on hairless MIDI without touching the piezo. Maybe something is wrong with the soldering.
Could anyone help me with this?