Jesus in His humanity was not omniscient.
The Persons of the Trinity share one mind and one will. Since the Incarnation, the Son has assumed a second, human mind and will (and body and soul).
So it is not possible for the Persons of the Trinity to keep knowledge from each other or disagree, because they share the one divine mind and will. But it is possible for the human mind of the Son not to know something that His divine mind knows. (For example, Jesus most likely never learned Chinese in the human manner, and He certainly was never taught yet-to-exist languages like English. But as God He knows all human languages.)
Now, exactly how all that worked in Jesus’ personal experience is something theologians have debated ever since. Was He sometimes truly ignorant unless He chose to tap into His divine knowledge, or did He always know everything but sometimes spoke from a human perspective? Only He could tell us.
Usagi