Why Nanatsu no Taizai members didnt recognize each other at first in season #1?
For Meliodas and Ban, I am pretty sure they did recognize each other. It's just the surprise they get after 10 years.
For King and Ban, King did recognize Ban, and Ban to King. King's hate of Ban for Elaine's death was the reason why he acted like that. Ban just doesn't care.
For Gowther, the others have not really seen him in the flesh before that moment, he has only seen King in his old fart form, and he did recognize Ban and Meliodas.
Do point out any episodes that prove otherwise. It can help move the discussion going.