What are you talking about? Sakura needed a specific amount of chakra gathered in her forehead to manifest the seal.
If you followed the topic you will understand, but if you choose to believe blindly with you fav then there is no need to argue anymore.
Before the cloak she was not able to perform the seal. But because of the cloak she was able to take chakra from it and channel to her seal. So it will take sometime from that momment until sasuke arrives to activate the seal. That will be scenario 1, which is better excuse for sakura with nejii death and not using 100% her skill for the war.
Scenario 2 which is stupid because she save her skill to show off to sasuke. Note that the time btw neji death until sasuke arrive is quite short and not an excuse for her seal to not activate back then.