Are they getting the CC Info is incorrect message on the checkout failure page?
the only way for the checkout failure page to load is if the transaction goes through to the gateway and gateway fails the transaction.
Where it is only happening for some people, I would guess that it has to do with the way they are entering the credit card info.
they could be entering the credit card number using dashes:
or could be entering the expiration year incorrectly as a 2 digit year: