Okay, after some more experimentation, I've got the following:
All of the default, preset voices appear to work fine regardless of how many effects are involved (between 2 and 5). Though in the process of testing that I accidentally changed the "Robot" voice away from its defaults, can someone tell me the original preset value of Robot's Echo effect?😒
The custom mod I was having trouble with was a 4-effect string - Pitch 0.6, Distortion 100% at -32 dB, Low Pass -1600 Hz, and Amplify 250%. It was otherwise identical to the "Ogre" default except with a lower Pitch and higher Amplify. Altering the Pitch quality up to 0.7, though, and now the mod seems to work just fine. So I kept tweaking, and it appears a Pitch value of 0.62 or lower causes the mod to stop detecting input of any kind. Verified this with another custom that was nothing but Pitch, and at 0.62 the input stops working. At Pitch 0.63 or higher it seems to have no issues.
So thanks, Yusuf, I know what the specific issue was, though now I'm baffled as to the 'why'. Is this a problem with my microphone and/or natural speaking voice, that lowering its pitch too far means Voxal can't "hear" it at all?