I think I have discovered the problem. When I unchecked "Process only new science files written after pressing Start button" in Preferences, the science files appeared.
The interesting thing is that I have never run these files through DP before. However I did import them as an image sequence from the AstroImageJ main screen earlier, before I tried to run them through DP.
Now the science files show 336 as would be expected, but the calibration files show 0. AstroImageJ must know that those calibration files have already been used and have created master calibration files.