Every board has been tested with a Pi before shipping, so the only thing I could think of is perhaps the shift register is not getting signal from the RPi correctly. Not sure what caused this, but that’s the most likely cause. The RPi sends signal to shift register through 4 pins: NOE (output enable, active low, usually pulled high, but is set to low when a program is running), LAT (latch), CLK (clock), and DAT (data). In particular the NOE pin should be set low when a program is running, otherwise the output will be disabled and hence no voltage.
Could you post a high-resolution picture of your setup?