My score does have the decrease marked, but I'm still finding that tricky, I struggled to bring out the melody, too - my 'a' finger tends to be all or nothing at the moment!.
Well I always like to stress the extent to which all these studies should be regarded as musical entities first and foremost ... in which case we can say that for yourself right now, the technical thing of 'a' finger control - including the legato falling-away effect - is an important musical requirement!
As wchymeus points out, really hearing the musical effect - the bass-treble dialogue - is the thing. Technique-wise for a moment though, see if there is a tendency for the 'a' finger to be restricted by the 'm' finger which precedes it, and think about allowing the 'm' to return back out of the hand as the 'a' prepares for its stroke. It can be helpful to do the 'a' both rest and free stroke, the former being the less common approach in the modern age, but encouraging awareness and control.