The light cube's battery may be low. Each light cube contains a 1.5 volt alkaline battery, type LR1 or E90. If the voltage drops below 1.3 volts the light cube's behavior can become unreliable, and as it drops further, it will lose the ability to communicate with Cozmo.You can check if Cozmo can connect to his light cubes by going to the Cozmo app on your phone or tablet (exit SDK mode if necessary) and clicking on the Settings button in the top right corner of the screen. Scroll right and you'll see a Cube Status section that shows which cubes Cozmo is talking to. Click on the "Refresh Cubes" button to see if he can make contact with all of them, or if some cubes are dead.Even if Cozmo can connect to a cube, the cube could still have a low battery.  Calypso will warn you about this by displaying a flashing "cube low battery" warning at the bottom of the world map when in execution mode. If you don't see any warnings, all the cubes Cozmo is talking to are at 1.3 volts or above.For help on replacing a cube's battery, see the Anki Customer Care Center article How to Troubleshoot Cozmo's Cubes.