several possibilities, all many times mentioned in this forum:

1.you did not yet load this subscription on the card, as I guess it is brand-new for you

2.you did not clearly state for what means this check-in was: train/NS other? Bus?tram? There are sometimes clashes on this

3.a faulty check-mast-happens occasionally with non-NS transporters

As always: via mijnNS you can check what has been registered and what status is on the card. Have to open via site, cannot do via app.

Coma back if you have tried all this and it still remains

Thanks for answer. I meant it for NS trains. I subscribed it via mijnNS and I linked my OV chipkaart to mijnNS. Should I do anything else on OV chipkaart too ? 

If you order NS Flex for an existing OV-chipcard, then you need to load NS Flex on the card at a ticket machine. There, hold your card in front of the card reader and choose the option to collect an order (in Dutch it’s called “Ophalen bestelling”). Then follow the instructions on the screen to accept the order (in Dutch: “Akkoord”) and hold your card in front of the reader again.

Thank you very much. Now I understood the missing part. I will try it.