Before your eyes…an answer.
This is a pretty straightforeward theory. Why, oh why, did Sun not “flash”? For the same reason that Ben, Locks, and Frank did not flash…they did not bring everyone back. Aaron, Walt, and especially for Sun, JiYeon were not “brought back” to the island…plain and simple. Like Faraday, I have no scientific answer why the people returning have an influence over the concept of island time, but they do. There is something missing we are not aware of, be it Jacob, Smokey, or with the history of the island itself. Regardless, Hawking said there would be consequences if they all did not return, and I think we are seeing those now. She also said that the amount of returning Losties would do for now. I, imagine a return aboard the S.S. Desmond, possibly with Des’s son, and Penelope if she is still…alive. And if not, what will happen if Des brings Penny to the island in death? Will she have a Christian/Locke like awakening?
Regardless of the details, the question is constantly asked here, and I practically guarantee that this will be the cause of all members of 316 that were supposed to flash, not flash, and the solution to Sun, Ben, Locke and company getting to the time period of 1977.

I don’t know. You’re saying that because not everyone came back, Sun didn’t flash? Kate didn’t bring Aaron…
I think that Sun didn’t flash because she has work to do on the island in 2007. It’s all part of her “long journey” that Christian has started telling her about.
Ben and Hawking both said there would be consequences and it would be unpredictable so to me this is just about as much sense as you can make out of it.
But your wright there is something missing and what ever it is it deciding who goes where, why and when as they enter that Island window.
I think the key to knowing why Sun didn’t flash is the same as why Ben – and maybe Lapidus – didn’t.
– Work to do in 2007?
– A previous self exists on the island in 1977?
– Didn’t bring someone back?
It’s obviously more than the writers trying to prolong the (hopefully alive) reunion of Sun and Jin.
My feeling on the matter is that Sun did not flash at least in part because she has become a major player in the Ben versus Widmore shenanigans. Effectually, her power and influence as general manager of Paik, and what she has chosen to do with that power has made her a third player in what wil occur between Ben and Locke, somehow. While she has the obvious journey to save her Mansel in Distress, lacking blame for Jack or even Ben at this point doesn’t seem to relieve her of a need for blame -beyond her father- and I think she may blame the island itself.
Children returning, children returning, Children returning, Children returning, Children returning, Children returning.
and no, I dont know why…
AES, you said something wonderful in here that made me think. (Whether or not it’s really that important though doesn’t matter.)
In my mind, it’s a strong possibility that Hawking knows more than the simple explanation that Locke could be a proxy for Christian, and that’s why he needs to go back. She probably knows about the ‘resurrecting’ type powers there.
Could it be possible that Penny did die and that Hawking will fill Desmond in on a little of the “how the island works” type of info that ultimately motivates him back there, with a corpse of Penny?
I have always thought that Penny dying could motivate him enough to get back to try to ‘undo’ something – but the thought of taking her back is actually a new spin to think about.
I know this wasn’t your point though. Oh well.
Maybe that is the very chance for Widmore to come back to the island, to take his daughter to be alive and maybe be judged by smokey…
We shall see. If she didn’t flash because the children didn’t return, then she will be stuck where she is until they get back and that could be 20 or 30 years from now. I don’t see Sun just hanging out in New Otherton for that long. Can you imagine how boring that would be? For her and us.