[SOLVED]Flashcard from example sentence issue (using Anki)

[Emanuel]

I'm having trouble getting the little + icon next to the example sentences to show that a flashcard has already been created from it.
The words clearly show when they already have a flashcard created with an outline around the + sign.
But for some reason this is not the case for example sentences.

Is this a bug, or a known limitation?

Thank you!

 

[Emanuel]

I've just figured it out a few seconds ago!
The checkbox for use for sentence duplicate checking wasn't checked...