Useless voucher

Firstly your staff are ***********, I received your so called voucher, thinking great stuff when I get my dog food that I usually get from west pack I will use the voucher and get something extra. Your staff tell me that I cant use the voucher there and only at the other stores. I asked a simple question that I got no answer for. I asked both the cashier and the manager why is this the case if I've only ever shopped at the pet side of west pack and submitted my details there how can I now not use the voucher there?. I've never shopped at the other stores that sell all the other stuff, just at the pet store. I got a dirty look and no explanation from your store manager.