Cannot pay online

I have been a customer for 13 years and have always paid my bill online with no issue until last month when trying to do so I am told to pay in person.  I have talked to customer service with no resolve it is so frustrating. I went ahead to a store and I still could not pay with my card. I was literally forced to pay cash.  The store rep saw no flags on my account. I thought next month it would be fine but same message again pay in person spent 30 minutes talking to another rep saying I need to store my payment blah blah  but that is not true I even gave a card number over the the phone and they tried to pay on the back end it did not work so I was told. My card is valid and has available credit, so I can't figure out what is going on. I do use different cards and make several payments however it is not fraudulent. I need for this to be fixed. Now I have to go to a store again this month and pay cash, this is so not cool. and it doesn't help when the rep is trying to get me to sign up for 5 year price guarantee. I would like to pay my bill online at my convenience.