Std bank doesnt know why my credit card keeps declining

Have been struggling with my credit card for the past few days. It seems like transactions under R1000 succeed, But anything over R1000 declines. Im now behind on some payment and will most likely have to pay additional fines. Ive been in contact with their agents multiple times and they have no idea why my card doesnt work. They just keep saying to go and try and swipe it again. It hasnt been swiping for the past week! Why will it suddenly work if you didnt make any changes on your end!?