You may have plugged in the Guitar Rig effect plug-in in Logic in a mono audio object and then switched the object into a stereo object.

This will not be succesfull.

You first have to switch the audio object to stereo, then plug-in the Guitar Rig Plug-in.

If done otherwise, after switching to stereo the plug-in will remain a *mono* input plug-in, as it was instantiated as such. Logic does not convert it to a stereo plug-in automatically. Therefore the plug-in will not be able to receive the input from the right input (where usually the control signal is present).