Originally posted by Darth_Nefarus
Actually something like this was posted before the movie came out. I believe GL claims that Sidious' connection to the darkside is so strong that he is as blind as Yoda was in the PT. He never was connected to the lightside, but Vader was.
That's how he immediately knows Obi-Wan is alive in ANH, and how Vader, not Sidious manages to sense Luke in ROTJ.
There was a lot more detail but this is the just of it.
This makes a lot of sense. Although I believe that Vader could sense Luke because it was his son.
As to Sidious not sensing Luke, he is still human. He successfully forsaw practically everything from before Episode I to late in Episode VI. He didn't see his own death for some reason. I believe it was because he was convinced Vader was completely evil, and he could not sense the lightside still dwelling in a small part of Vader.