Unprofessional & Slow to reply!

I received a gift voucher, as a gift. I used it once and still had credit left, Then wanted to purchase for the second time and the cashier said the voucher is invalid! This happened at the Ottery store, so why didn't they cashier then print a receipt to show how much credit is left??? They are so unprofessional and not of much help.

Customer service emailed me yesterday and I still haven't received feedback! Can someone assist because this is pathetic service!!