I can only play back to tracks from my D A W project into the U track 24.
Only channels 1 and 2 seem to play back.
I thought I should be able to playback 24 tracks and listen to all 24 tracks of the other U track 24 headphone output.
What am I doing wrong
Yes you are right.
But you can create a stereo downmix to CH1-2 from the rest channels 3-24, in this case you can have 22 channels from your DAW with stereo downmix.
The headphones use the same output like the Main out 1-2.
If you have 1-24 channels input on the D-Sub connectors the downmix done internally in the uTrack and you can listen in the headphones.
With the mixer panel on your PC you can see two fader to adjust the D-Sub inputs and DAW output (CH1-2) levels.