Infact they (shias) themselves dont follow their narrations about 12th Imam.
12th Imam will appear only when they (shias) stop annoucing his coming.
Kitabul ul Ghaybah of Numani,
Chapter 18: FORBIDDING TIMING AND NAMING ALQA'IM
Narration #1
Ahmad bin Muhammad bin Sa'eed narrated from Ali bin al-Hasan from al-Hasan bin Ali bin Yousuf from Muhammad bin Ali from Sa’dan bin Muslim that Abu Baseer had said: I said to Abu Abdullah as-Sadiq (s): “Does this matter (the appearance of al-Mahdi) not have an end so that we relieve ourselves?”
He said: “Yes, it does. But you (people) announced it; therefore Allah put it off.”
Narration #15
Muhammad bin Ya'qoob narrated from al-Husayn bin Muhammad from Ja'far bin Muhammad from al-Qassim bin
Issma'eel al-Anbari from al-Hasan bin Ali from Ibraheem bin Muhzim from his father that Abu Abdullah as-Sadiq (s) had said when the (Abbasid) rulers had been mentioned before him:
“People perished because they hastened this matter (deliverance). Allah does not become hasty just because that people urge on their matters to be achieved. This matter (the appearance of al-Qa'im) has a precise time that it must reach. If it reaches its time, it will occur neither an hour sooner nor an hour later.”
And when he (12th Imam) will finally come, most of the shias will leave shia'ism and will apostatize.
Same book,
Chapter 21: The Shia when al-Qa'im appears.
Narration #1
Ahmad bin Muhammad bin Sa'eed bin Oqda narrated from Hameed bin Ziyad from Ali bin as-Sabah from Abu Ali al-Hasan bin Muhammad al-Hadhrami from Ja'far bin Muhammad from Ibraheem bin Abdul Hameed from someone that Abu Abdullah as-Sadiq (as) had said:
“When al-Qa'im (as) appears, those, who have thought themselves as his followers, will apostatize and those, who are somehow like the worshippers of the sun and the moon, will believe in him.”