Thank you for such a prompt reply. It's not all the RealTracks that are causing this problem. Here is the detail you asked for.

I'm using:
Version - 2021, build 804
Audio Settings - ASIO Focusrite USB
Playback Device - Focusrite Scarlett Solo

Like I said, it's not all the tracks, only the occasional RealTrack. Below is the memo for a song chosen from the list. The track with no output is the Guitar 2 track which is:

RealTracks in style: ~~522:Guitar, Acoustic, Fingerpicking Ev 120



****** Song Summary *************
Title: Country Ballad with 2 Fingerpicking Guitars
File:NashvilleBrushesEasyBluegrass_140_Style.MGU
Key=E , Tempo 140, Length (m:s)=3:25
No intro. 116 bar chorus, from bar 1 to bar 116. Repeat x1 chorus
Melody has 827 notes, Melody harmony is < no harmony >(0)
No Soloist track.
Song is saved with Volume, Pan, Reverb, Chorus, Bank0,
Style is _FPBNTPS.STY (Country Ballad w/ Fingerpicking )

RealTracks in style: ~~519:Bass, Electric, Pop HalfNotes Ev 120
RealTracks in style: ~~621:Pedal Steel, Background Easy Ev 136
RealTracks in style: ~~1677:Guitar, Acoustic, Fingerpicking CountryBrent Ev 120
RealTracks in style: ~~522:Guitar, Acoustic, Fingerpicking Ev 120
RealDrums [in Song:NashvilleBrushesEasyBluegrass: a: Sidestick, Brushes b: Brushes, Stick

*******************