Aerosoft A320 - FSUIPC and LUA

Hello.
I am trying to map the main functions of the Aerosoft A320 to my Keyboard using FSUIPC and Lua-Scripts.
I am having some trouble with it, because some functions work and some dont.

For example, the FD can be turned on and off without any problem, but I cannot toggle it. 

 

function Autopilot_FD_on ()
    LVarSet = "L:AB_MPL_FD"
    ipc.writeLvar(LVarSet, 1)
    SmallOverheadPushButtons ()
    DspShow ("FD", "on")
end

function Autopilot_FD_off ()
    LVarSet = "L:AB_MPL_FD"
    ipc.writeLvar(LVarSet, 0)
    SmallOverheadPushButtons ()
    DspShow ("FD", "off")
end

function Autopilot_FD_toggle ()
	if _tl("AB_MPL_FD", 0) then
		Autopilot_FD_on ()
	else
		Autopilot_FD_off ()
	end
end

event.flag(0, "Autopilot_FD_off")
event.flag(1, "Autopilot_FD_on")
event.flag(2, "Autopilot_FD_toggle")

It's the case with many more functions. Strangely, I found one toggle function which is using the same syntax as above and it works. Why does the FD toggle function not work for example?

I took this code from the LINDA forum.

Thank you in advance. :)