Booking does not accept different credit cards from guests for online booking

Hi, since a few days I saw a big decrease in bookings and then some guests who arrived at the door told me and showed that they tried  to make the booking, but booking.com says about each credit card that it is not valid or they should try a new one. Firstly I thought it is true, but then another guests called me and told me that they can´t book online because booking won´t accept any credit card, has anyone experienced this before? Thanks