This problem may be linked to the fact that you have written very close to the grey area shown in the image below.
When your writing is in this area, there is a risk that the application will not be able to read the colour of the flashcard (which is encoded into these small shapes). When the colour of the front or the back of the flashcard cannot be detected by scribzee, there is a risk that they become separated.
We therefore advise you not to write in this area to ensure that the application detects the colour of the front and back of the scanned flashcards.
If you have already produced flashcards by writing too close to the grey area, use correction fluid to cover the writing. This will allow you to scan the front and back so that they match.