Self-test mode turns on each station for 10 seconds one after another. Before measuring the voltage of a particular station, make sure its number is displayed on the LCD.
Also, make sure you are measuring the voltage between the common wire (COM) and a station. If you still can't get the correct voltage, you can check the shift register output voltage of each station. When a station is turned on, the corresponding shift register output pin should be about 3.3V. Finally, check if you've soldered the triacs correctly.