When you have a Dal Voordeel subscription you need to check in en out with the OV-chipcard on which the subscription is placed. You can not buy off-peak tickets in the app, not now and not before. If you did so before, you probably bought Samenreistickets, which are meant for other people who travel together with you, it was never allowed for the subscriptionholder to use those tickets.

Thank you for your answer! It was not clear at all this thing. It was not written anywhere that it was not possible to purchase a ticket in this way… or at list I haven’t seen it. I usually prefer to buy the ticket not through the ov-chipkaart because I don’t have always 20€ balance on it, so it’s easier through the app. So is this not possible in any way? Thank you. 

It is definitely not possible. The only way to get the 40% discount with a subscription is to check in and out with that subscription (so with the OV-chipcard). And with NS Flex Dal Voordeel you don't need any balance on the card since all costs are deducted automatically once a month from your bank account. You will, receive an invoice once a month with the costs for the subscription plus all travel costs. All you need to do to get the 40% discount is check in during off-peak hours. 

Ok! Maybe I’m just confused! Thanks a lot!

off-peak 

Of course it is off-peak. No idea why I typed peak. Thanks for the correction. It is now corrected in my earlier reply. 

Request a refund of the overpaid amount using the form on www.ns.nl. 

This website is only in Dutch.

Next time, always check in and out with your transport chip card.

You can choose for English site at the right top.