Are you assigning to a control or a keypress? Does the keypress (z key?) work?
This thread may be helpful: http://forum.aerosoft.com/index.php?/topic/38298-cant-disconnect-autopilot/)
There is also a Lua script for controlling the Aerosoft A320 autopilot functions available from a link in this post: http://forum.aerosoft.com/index.php?/topic/85325-airbus-a318a319-lua-script-for-fsuipc/ and further information here: http://forum.aerosoft.com/index.php?/topic/68729-using-joystick-to-control-autopilot-functions/
Also, for future reference, it is always a good idea to attach your FSUIPC5.ini file for assignment/calibration problems.
Cheers,
John
EDIT: looks like the lua scripts from those links are no longer available. Instead you could download the lua scripts from the linda package here (or look for a later/updated version) : https://www.avsim.com/forums/topic/451948-aerosoft-airbus-module-44-v3/. You don't necessarily have to use the whole script, but it might give you some insight into how to get this working (i.e. its using the lvars AB_AP_AP1& AB_AP_AP2 to turn autopilot 1 & 2 on/off (with parameters 1/0 respectively)