Nowadays, Weekend Vrij does not automatically include 40% discount outside peak hours on workdays. This option has to be enabled explicitly:
 

It sounds like you didn't add this option to your season ticket, in which case the discount was rightfully not applied. If you did add this supplement, then note that the discount only applies if you check in during off-peak hours. Checking in during peak hours means you pay the full fare, even if your entire journey takes place during off-peak hours.

If you still feel you should have been given discount, then you should contact customer service directly to sort this out. This can be done by phone; they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook. Note that it can take a few days to receive a reply if you contact them online.