Science Buddies: "Ask an Expert"

Hi there,
I have assembled an obstacle avoiding robot using the circuit instructions from Science buddies, but what happens is that if I press on a lever, the opposite wheel does not move backward. only forward (the wheel on the same side of the lever stops). Could you tell me what is going on, as I haven't seen this kind of situation appear in the FAQ section on the page. Thank you very much.

Hi, some troubleshooting ideas

1. Swap the left and right bump sensors to test if the left one is working properly.
2. Go over each connections to make sure they matches the wiring diagram.
3. Disconnect and reconnect each connection.
4. Use a voltmeter to check each the connection path.