From the A320 FCOM:
Autobrake activates:
- When the ground spoilers extend, for LO and MED mode.
- When the ground spoilers extend and the aircraft speed is greater than 40 knots, for MAX mode.
Therefore, if the aircraft makes an acceleration stop and begins to decelerate when its speed is under 72 knots, the automatic braking will not activate because the ground spoilers will not extend.
Moreover:
- Autobrake "LOW" activates with 4 seconds delay and decelerates at 1.7 meters/second2
- "MEDIUM" activates with 2 seconds delay and decelerates at 3 meters/second2
- "MAX" activates with zero delay.