I asked the following question about 3 weeks ago.

Sometimes tablature just won't show. Example, I just did a bass track. All the notes show correctly but only notes played on the "G" string appear in the tablature. I.e. the bass tablature is not showing notes on the "E", "A", or "D" strings.

Just discovered the answer tonight:

Select the miscreant track. Go to Edit, select Rechannel.

I think the explanation is that the instrument was recorded on a single midi channel. Selecting EDIT>Rechannel re-records each string on a separate channel. For some reason tablature requires separate channel for each string.