I'm trying to map a couple of dials in SynthMaster One to my midi controller using ACT. I know how it should be done and have made it work with a variety of other plug-ins (to prove a point) and yet running the Learn routine is ignored. 
ACT knows SynthMaster is there and has automapped the plug-in parameters based on who-knows-what and the midi controller will move the mapped parameters so I know they are talking to each other.
Any thoughts on why it won't work?
 
Grum. 

I have just tried with SynthMaster One Demo and ACT learning was fine (I have obviously have not tried with all possible controls).
But first of all, why you want ACT instead of in-plug-in MIDI learn?
 
I have seen at least one plug-in recently, which allows to learn, but only once... any future attempts produce the behavior you describe, after moving parameters and knobs in ACT Learn mode, finishing ACT Learning produce no effect.
 
AZ ACT Fix (http://www.azslow.com/index.php/topic,297.0.html) can help to "clean" current mapping (and solve other problems).