Just a quick tip to help anyone with this issue. I hope it saves you at least a few minutes of frustration…
I found that when midi items are included in a Folder, and you attempt to use Pro-C on the Parent Track (in this example, attempting to add Compressor the the ‘Instruments’ Track) …
Pro-C would act more like a limiter – at least, the ‘Gain Change’ meter would not move – and the compressor just flat-lines.
This is due to Midi being enabled.
To disable, click on the ‘Midi Learn’ drop-down, in the bottom-left corner and deselect ‘Enable MIDI. (Must be done when track is stopped).
Note: Sometimes the ‘Enable MIDI’ option appears to reactivate itself – and will need to be turned off manually again, when track is stopped (ie. when switching between presets whilst track is playing the Midi re-enables itself).