I was using this plugin, and I had a MIDI track I was routing through it. It was all working well, and then for seemingly no reason, it failed to make any noise. It was clear that it was receiving MIDI, so that wasn't the problem. And then I plugged a MIDI keyboard into it, and when I played that, it worked! How can it play live MIDI but not a MIDI track? Any help appreciated.

At the risk of being no help at all let me say that this happens to me in X1 not infrequently.  And I have no idea what causes it.  Occasionally playback will simply resume if I stay and continue working in the project.  But more often it requires that I close and then reopen the project to regain playback.  And it's not just Lounge Lizard.  It can happen to almost any VST in my experience. 

I'm going to resurrect this old thread in case anybody has any new insight into this problem.
 
I'm using AAS Lounge Lizard Session in X2a - as an instrument track. After a certain amount of editing in the PRV, the sound always stops. If I then freeze and unfreeze the track, it's OK again (for a while). It never stops unless I've been editing in the PRV.
 
Does anybody have any idea what might cause this, or how to fix it? I haven't noticed it with any other soft synth. I've re-installed and re-authorized a couple of times.

This happened to me - I have a suspicion that it's connected to the demo version if you installed that before installing the paid-for one. I contacted AAS for advice at the time, but didn't follow up because I bought the full version of EP-4 which works properly. Sorry I can't be of more help, but at least you can tell AAS support that you're not the only one with this problem with Lounge Lizard session

One way I used to get sound started up again was to decrease the number of voices on the GUI, then restore it. This would get sound back for a while, so I tended to do what you're doing, and freeze while I was getting output. Not a great way to work!

Thanks for the info ... good to know it's not just me.