Board Thread:Oblivion/@comment-9716469-20130519212835/@comment-25757480-20160222214645

Tiber Septim was blessed with the blood of a dragon (in other words, he was Dragonborn. After ascention to godhood and becoming Talos, his grandson, Pelagius Septim, took on the throne and became emperor. However. he was assassinated before he produce an heir, so in other words, the bloodline of Tiber Septim ended that day. Since then, Tiber Septim's nephew became the emperor, and all of his heirs. Since Tiber Septim's dragon blood is found in Martin, Martin cannot possibly be related to Dragonborn. The Amulet of Kings doesn't "scan" for dragon blood, but rather requires the "blood of a dragon". This can be taken metaphorically, Tiber Septim was of dragon blood, and the Septims are related to him on a more human level, perhaps the Amulet of Kings only works on those related to the Septims with the DNA of Tiber's father, so basically, blood of a dragon, but not literally dragon blood.

TL;DR: Martin aint got no d-blood dat amulet works cuz martin relates to old t' sept' on a mo' human level.