Obviously annoying question: Did you connect the power expander battery? Does the LED Status indicator on the expander show stable green? (not blinking, not other color)
Have you properly wired IN (to the cortex, in the center of the expander) and OUT (to the motors, closer to the battery cable) ports?
Have you locked the "PWM lock" (the plastic slider over the whole expander body that makes sure the cables won't detach)?
Are all your cables properly oriented (I know the cables are keyed. But the universe around us sometimes doesn't care and we have had the same problem last week with cables plugged in wrong)