This is because there are no target candidates with remaining frames. Any target with a checkmark is complete (and will remain complete even if you add frames to it). To add it back to the sequence, right click it and choose the option to mark it as incomplete.
Ahhh. But it wasnt complete. As you can see from the image, it still has frames to shoot. These were not added later.
It did however reach a time limit and stop on the previous night. But it didn’t complete.
Thanks
David.