Sometimes it's a matter of transposed numbers when entering the info. Other times, if the guest booked their car first, the car company holds an excessive amount of money on the card, so if they were 'close' they are now over.
I am in a sympathetic mode right now because I just bounced 4 checks. It was an error on my part in transferring the funds, I did not complete the transaction so the money never transferred. So, for 3 days, my checks bounced, adding NSF fees to every one of them, meaning I had less and less money everytime one of them bounced. It's very easy right now to get in deep very quickly. It was $120 in fees from my bank and then another $55 in fees from the folks whose checks bounced.
So, if a payment isn't applied to a cc on time, the card holder has fees tacked on and their interest rate jumps and that can be if they had a problem with a completely different card!
My son had his cc declined in a foreign country, nothing wrong with the card but his cc company thought the card was stolen so they declined it.
Someone on here had her card declined at her local grocery store because her husband was using the joint card in another country at the same time (go figure). So the cc company declined HER use of the card at home (again, go figure).
Gomez used our new card to buy something online and used the wrong zip code (which could also be a problem with your guests) but the charge went thru. When he tried to use the same card in another country the following day it was declined (much to his embarrassment as he was with family at the time).
OTOH, yes, people do have problems keeping track of what's on their cards at any given time..