Why didn’t Sauron know that Frodo had the ring when he put it on in Bree, or that Bilbo had it when he put it on at the party? And why didn’t he know that Sam put on the ring in Mordor? Sauron percieved it when Frodo put on the ring in the cracks of doom, and he knew exactly where Frodo was, so why didn’t Sauron just send the Ringwraiths to attack Sam, or even just send them when Bilbo used the ring numberous times in the Shire?
He wasn’t looking for it at the time. When Sam put it on, Sauron was convinced that Aragorn had it.
Sauron only knew that the ring was in the Shire and that a hobbit had it after capturing Gollum and torturing him.
In the book, it was ok to put the ring on in Mordor in the beginning, Sam wears it for quite some time after Frodo is captured and Sauron doesnt sense it. In the movie though the ring grew in power as it got closer to Mordor and Mt Doom and so Frodo knew not to use it, its the same in the last couple chapters in the book also.
Hope that answers your question even a little bit. Im sure someone else wil be along with a more detailed answer, but thats about the basis of it I think.
I didnt read the question properly, so my answer doesnt make much sence, sorry!
Each of the times that Frodo (or Sam) used it, there was a risk. If the Eye had been directed in that direction at the time, Sauron would have seen it. It’s largely luck that he wasn’t looking in those directions at the time.
But when Frodo used it at the Crack, he claimed it for himself (or at least, tried to). That got Sauron’s attention right quick. Even if he hadn’t been looking that way before (straight out the window of the Eye), he would have when Frodo claimed it.
A hobbit without the Ring is basically invisible to Sauron (he might be able to see someone of power like Gandalf or Galadriel, but not a simple mortal). A hobbit wearing the ring would be visible, but only if he was looking at him. But what Frodo did at the Crack of Doom was the equivalent of jumping up and down and shouting “Hey, look over here!”.
When Frodo put the ring on in Mount Doom, he was claiming it for his own. This is the main difference, I believe.
dang…here I thought I would be able to put my LOTR expertise to use, but I believe previous posts have answered the question. The fact that he claimed it for himself def made Sauron ‘notice’ it. =)
-Mith