1. Auto close mode works on next sequence:
1.1. Barrier Lowers
1.2. Car parks
1.3. Car stays on the barrier for more than 60 seconds
1.4. Car moves from the barrier in a continuous motion
1.5. The barrier auto-closes with the specified interval that is always 10-15 seconds
If one of these conditions is not met barrier will not activate auto-close.
Also sometimes happens that car is not parked over sensor and does not activate it.
You can test auto-close feature by putting object like carton box or t-shirt over sensor to check it manually.