I think there was a limit as to how many times that little amount of the cure could potentially pass forward. Kat got the cure and it was flowing in her blood but it was just a little vial and Silas had to drink pretty much all of hers to get enough of the cure from her body. So It did seem a little bit of a plot hole that Amara had the cure after only taking some of his but then she was the walking cure like it passed from person to person. But then when she was dead it was maybe just to far gone like to much diluted to keep passing it on. It was kind of dropped plotline wise as it had been the main drive for so long it was a little confusing how that ended. Silas died a witch Stefan threw the knife at him, Amara had drank the cure from him and then stabbed herself so she died human as tess made Bonnie the anchor so they aren't together in heaven as he would be stuck as a supernatural on the other side or perhaps like Pearl and Anna if the resolved their business which was to die and be together then that was maybe enough to get them to peace. She gave up the idea of taking the cure as she was accepting her life as a vampire. Although as we see with kat it may have had an adverse affect on anyone taking it that was older than a few years. I know she was aging and dying because silas sucked it out of her but then without it time caught up fast and she would have been a walking target had other vamps found out the cure was human and walking around.