[FONT=Palatino Linotype]Salam alaikum.
Shias claim that there was a nass on 12 aimma from prophet (sallalahu alaihi wa ala alihi wa salam).
Some of them even claim that he (sallalahu alaihi wa ala alihi wa salam) named them name by name.
On other hand we know from history that Imam Jafar first showed his son Ismayel as a successor and only when he died, he showed Mosa.
Al-Hadi first showed Mohammad as a successor and then when he died, he showed Al-Askari.
Question is, if there was a clear nass, or even like some shias claim ahadeth where all 12 aimma were foretold name by name, why twice aimma showed other one as a successor?