It's not uncommon that journeys made with other operators such as GVB take a while to be processed. Even though three of the journeys you made took place before the previous invoice's date, it could be that you haven't been charged for them before. Are you sure they were on the previous invoice already?
If so, you should contact NS customer service directly to have them correct this. Usually, this is most easily done by phone, as 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 X (formerly Twitter) or Facebook.