A320 FCU: variable AUTOPILOT ALTITUDE LOCK VAR vs. displayed altitude

Good morning
Sorry to disturb you, I am quite new to this subjects even if play with FS for more than 20 years.
Ilove to fly the A320, I even have the oportunity to be 8 hours in a real A320 simulator but now I am trying to use Mobiflight and FSuipc to make some harware controls and I have been able to change the AP Heading and Altitude, and so on but when in Altitude I change the value, in the real plane the Autolilot will not start controling the new Altitude and we need to Pull (or push for management ) the Knob to that start controling. When I change the Altitude using Mobiflight and AP_ALT_VAR_SET_ENGLISH the AP starts to change the Altitude at once.
I believe you had the same problem but the variable you are spoking about (AUTOPILOT ALTITUDE LOCK VAR) I dont’ know how to use it. I satrt installing the SDK but it seems to complex for me. Can you help me some how.
Best regards
Joao