Several things could be the problem here, but we don't have enough information to determine which is the case.

Firstly, have you just recently ordered NS Flex for an existing OV-chipcard? Then you need to activate NS Flex or it won't work. This is done by going to a ticket machine, holding your card in front of the reader, selecting Ophalen bestelling/Pick up order and following the instructions on the screen.

If that's done and it still doesn’t work, do you experience this problem when you travel by train or by tram, bus and metro? NS Flex actually consists of two different products: travelling on invoice in trains and travelling on invoice in bus, tram and metro.

If you experience this problem only when you travel by bus, tram or metro, it could be that this part of NS Flex isn't activated. This is intentional if you have a season ticket for bus, tram or metro, in which case you do need to travel on balance to benefit from that discount. But if that's not the case, you should contact NS customer service by phone, chat, X/Twitter or Facebook to have them enable it (which you then need to confirm again by picking up the order at a ticket machine).

Moreover, you should make sure the balance on the card is at least 0.00 euros. Some companies won't allow you to check in when it's negative, even when you travel on invoice.

If this all doesn't seem to be the problem, then contact NS customer service to have them take a look at it.