In order for Track Log to record automatically, make sure you have checked the following:
1. Verify you have a GPS source. iPhones and cellular-capable iPads have built-in GPS chips. If you have a WiFi-only iPad, an external GPS receiver will be needed.
2. Open ForeFlight and go to More > Settings and verify Enable Auto Start/Stop is selected.
3. Ensure ForeFlight Mobile is open and running in the foreground before takeoff. The Track Log will not record automatically if ForeFlight is not open and displayed during the takeoff phase.