I have been using the Navigo Decouverte pass for a couple of weeks, buying a weekly pass each time. I just tried to refill it with a monthly pass at the "recharge" machine, using a credit card. It denied my request each time, but let me refill with another weekly pass without any trouble. (It's not a problem with the limit on the card--that much I know.) Should I have used cash instead of trying to refill with a credit card? Why wouldn't it let me change to a monthly pass?
I'm taking a guess here, but it's one of two situations: 1) there was a conflict trying to add monthly top-up on a Navigo which still had some weekly subscription time left, 2) if the card is several years old, the card is nearing it's expiration and wouldn't allow a subscription beyond it's expiration.
Navigo cards are normally good for either weekly or monthly top-ups, so this is quite strange to hear.