Why does walmart have such terrible service?

i went to walmart, purchased stuff worth 70 dollars and at the cashier i swiped discover credit card. Cashier said, "declined". Ofcourse that was not true because credit line was 15,000 dollars in it. I said it cannot decline. I swiped again and he said again, "declined". I asked him to get manager. Manager never came. Then cashier said, "leave all the stuff here and go. " Manager confirmed its declined". After that I went to safeway and the same credit card worked. One week later I again went to walmart to purchase only $2 stuff. And then i swiped discover card and the cashier again said, "declined". I again said, "bring the manager". Manager never came and cashier said same thing, "manager confirmed its declined". Later when i got home i called discover card and they said it works and there is no problem.