I'd say the yeast is toast. I've got an eighteen month old batch of cider bottled. Absolutely no carbonation. I reyeasted a couple of bottles==nothing. Reprimed a couple==nothing. I think my yeast pooped out. And the alcohol content makes it difficult for a new yeast to get started. I don't like still cider so I guess I now have about 4-1/2 gallons of cooking cider.
Sorry to not be of any real help. I guess I could try making a full blown starter, dose each bottle when the starter was at high krausen, but I've been lazy. I've heard that could work.