I think you do need to replace the card, even when the office told you your card is ok. When it sometimes works and sometimes not, and gives wrong messages as well, it is typical. You are responsible for a working card and you need to order a new one.  When you have Flex and paid your bills a balance is not needed and when the card asks for it, it does not work properly anymore. Customer service may give a good advice, 030 7515155. 

Are these Connexxion buses?

Connexxion and Arriva.

I tried to check in on Lelylaan to a train today and it also didn’t work.

Ok.

I thought it maybe could've had something to do with what the bus driver told you and an email I've received from Connexxion (about the boarding rate increase from 4 to 8 euro). Even though Flex doesn't require a card balance, I've had issues in the past with Flex products and buses with a higher boarding rate. Especially Connexxion buses.

​​​​​​However, this is a while ago. If you have similar problems with the train and sometimes it works after a few tries (like with GVB) it probably hasn't got anything to do with that and Thom might be right about replacing your card .

Thank you for help :)

I ordered a replacement card-I’ll see next week how it’s gonna go.

With some transport companies (bus), there needs to be enough balance on a card even though it is not used!

Normally, the boarding fee for buses is 4 euro, and because the balance on a card can drop to minus 4 euro, it will go through.

As soon a a bus company raises the boarding fee to 8 euro (or even more for long distance lines), you will get the message about insufficient balance. You might want to top it up to e.g. 5 euro (which cannot be done at NS machines but GVB machines or supermarkets will work).