To expand on the answer that Fool Running gave, if you have postponed a check from Stage 1 to Stage 3 and Stage 1 is now complete, the Undo Postpone will not be visible. If you "temporarily" adjust a task in Stage 1 to no longer be completed, the option will return to show Undo Postpone and you can revert the check.
When you then recomplete the task, the check will remain in the original location. Here is a quick video showing the process:
Undo postpone