Transcript won't work if audio is just on CH2. Even if I have "mixdown" selected. It seams like there has to be audio on CH1 or it returns "No dialolgue found".
I think they have some work to do on MXF files. The following questions may help to narrow the issues down for you:
What is the source (if a camera, what specific model)?
In the Project Panel, if you right-click one of the clips, do you see Transcribe or Re-transcribe. Either way, click the one that is there. In the "Create transcription for source media" dialogue, do you see an option for "Channel selection" or only Language and Speaker labeling?
If you create a sequence from the clip, and then in the Transcript tab, 3 dots menu -> static transcription, does that work?