Invalid Gift Certificate Redemption Code

I am testing the Gift Certificate process on my new webshop.

All going well, however, when I try and redeem a gift voucher as payment for a product, in the cart (As the recipient of the gift voucher and GV code by email, I go to store, make a purchse, to to checkout, etc etc....I fill in the gv code and amount (it shows the amount of the GV I received)- the system won't accept the gift voucher redemption code and responds with

Invalid GC Redmptn Code.

What am I missing here?

Thankyou
Sandy