I had a similar problem, 2 tracks would play and export but the other 2 wouldn’t. Found out the 2 which wouldn’t play were routed to a different output .
Click on each track in turn and select the ‘e’ (edit channel settings) in the track info panel, in the top right hand corner of the new window are the input and output routing - ensure the output on each one is set to the same routing.
This may help or it may not
P