Receipt is getting planned again after manual firming

Hi Expert,

We are using the SAP_PP_SL001 Planning of Std Lots with Shelf Life. We maintained the shelf life data.

It planned the receipt element based on the that heuritics.

This is getting firmed manually and that is fixed in ds board too.

Even after firming in the when run the heurtics again this firm reciept is not getting changed, But new receipt is getting generated for the same requirement again.

This happen when we maintain the shelf life data. As soon we run the shelf life indicator from the product master property tab,Receipt is not getting planned again.

Means for requirement reciept is getting planned and manually firmed, But once we run the heuritics firm receipt is not get changed, But new reciept is getting planned for same requirement.

May i know the reason pls.