If your mod wheel isn't responding in MainStage, you'll want to check the following things:
- Did you assign the mod wheel in the “Layout” mode inside MainStage?
- Is your Mod wheel sending MIDI CC#1? This video is titled about the mod wheel and faders, but if you watch it through, you will see that it gets into showing what MIDI # is being sent, and also how to translate MIDI messages.
3. Does wiggling the mod wheel and then putting it at the 0 position fix the issue? (this is a common bug in the most recent version of MainStage)