The correction cannot take place at once; it is done gradually, step by step, since it is about the Reshimot (reminiscences) that remained after the shattering. It is the correction of the shattered vessels that must first each be clarified. Before that, there must be the clarification of the first vessel in order to build the next big vessel on top of it and then the next vessel, which is even bigger.
It is because everything is shattered, and the shattering can be corrected only in small parts, each part individually according to its level of shattering. The whole spiritual structure is made of discrete vessels that are connected by so many ties that they become one integral analog system.
It is like water that is made of separate drops, each of which is connected not only to the drops next to it, but to all the other drops, and not through other drops but directly. If you are in the world of Atzilut you are directly connected to Moses and to Rabbi Shimon.
So there is a difference in how the vessels are corrected after the shattering. It seems that we are connected to some friends by chance, thus joining some spiritual system and some Light. But by that, we activate an infinite number of ties. So the correction is gradual since it requires our clarifications.
From the 3rd part of the Daily Kabbalah Lesson 10/5/12, The Study of the Ten Sefirot