I have set up my cart to accept credit cards and debit cards.

I have successfully received a payment by Visa card, and have put through a dummy transaction with my debit card.

BUT I cannot process my own credit card!

I have a Mastercard, and after entering the details I an sent back to checkout step 2 with the message:

"The credit card number starting with was not entered correctly, or we do not accept that kind of card. Please try again or use another credit card.[cc]"

I have 2 questions.

1) Why can I not process my Mastercard?

2) In the error message received, the start of the credit card number is not shown. Why not?