This could be apparent in the app, the station screen, or if you don’t see the LEDs next to each module lighting up. 

It may mean your battery voltage is too low. To check the battery voltage, use the station screen interface to go into the Readings menu, then go down to the second reading with “[ ]” next to it (if you click the middle button, it’ll say “battery_vbus”). If the battery is low, you’ll see a number less than 3.55 V here. If the issue is being caused by a low battery, follow the instructions in The station screen is showing an “L” or “X” where the battery indicator is. What’s wrong?

If the battery voltage is not below 3.55 V, there might be a disconnect between your lower board and module base. Double check that everything is seated properly and the connections are in alignment.