Not connected to car engine?

Please open the device, remove the battery and check for anything loose inside.
Shaking it and it working sometimes, suggests that there is a loose or maybe a dirty connection.
If this is old, the battery terminal or where the battery connects may be dirty; cleaning can help provide a better connection. Try another battery to confirm the issue is not a flat battery.
It is unusual to have it work sometimes after shaking. Please check to confirm there is nothing loose inside.