RENFE online purchases have always been "challenging." Persistence does pay off. Watch out for Raileurope--they don't always show all the train options or offer all the discounts.
On our trip to Spain, we needed tickets Madrid/Sevilla, Sevilla/Granada, Granada/Madrid. Using RENFE, some routes did show up about 60 days in advance, others didn't get completely loaded until about 30 days in advance.
Also had lots of trouble getting our credit card accepted, but with patience and a couple of phone calls to USAA, it eventually worked out. It turned out we needed something called a "secure code" from Mastercard. It was easy to get, but it took a while to figure out that's what we needed.