The 1. file was not saved correctly, that is, it was not saved to the device after the annotation was set, so it could not be seen when it was opened again.
2. The file is damaged, some marks are invalid, and it needs to be repaired or reset before it can return to normal.
3. Notes are lost due to other people's or software errors, viruses in the computer, etc.
4. Annotation is hidden, or display annotation is not set.