unhappy customer

I retuned some goods I bought from Zando and they give me a Store Credit Voucher but I battled to use this Voucher , when I called them I was told that I cant use the voucher on the sales item, I ‘m surprise because this voucher is my money not the A Discount Voucher or a give away voucher, can you please Zando sort out my money or ORDER ***.BANK DETAILS HAS BEEN SENT