I did some tests of reset to factory defaults using the reset button on an Alix 2D13:
(a) Power on, wait 5 seconds, press reset button for 10 seconds, release
(b) Power on, wait until console is showing the FreeBSD booting stuff, press reset button for 10 seconds, release
Power on, wait until console is showing the pfSense booting stuff, press reset button for 10 seconds, release before/immediately you see "Launching the init system"
(d) Power on, wait until console is showing "Launching the init system", press reset button until "Reset button pressed" message is displayed, release
(e) Power on, wait a bit (5 to 60 seconds - no panic), while the left green LED is steady press the reset button, hold it until the 3 LEDs have been cycling a couple of seconds, release
(a) (b) and (c) do NOT result in a reset to factory defaults.
(d) and (e) DO result in a reset to factory defaults.
If you can see the console output, then (d) is a useful guide.
If you cannot see the console output (most likely scenario if you are needing to use the reset button), then (e) will guide you successfully.
Note that it is 90 to 120 seconds from power on until the 3 LEDs start cycling, so there is no rush to get the reset button held in immediately you apply power.