: : : Seems like a pretty likely scenario now that you mention it.
: : : Then again, he was kinda preoccupied with the white council.

: : : Gandalf

: : I think we talked a bit about this some time in the past.
: : The distance from Bilbo to Dol Guldur was, I think, around 100 miles or more. That might have played a part. Also, Sauron was still rebuilding his strength. That might have also played a part.

: : When Frodo and Sam are at the Cracks of Doom, they are closer to Sauron than Bilbo ever was. True, Sauron was distracted by Aragorn's army. Yet, Sauron only becomes aware of the ring when Frodo claims the ring as his own. Claiming the ring as his own,as Frodo did, was something that Bilbo did not do. Bilbo simply put the ring on and used its effect.It is possible that the act of "claiming" the ring had some effect on Sauron's spirit within the ring. Perhaps causing it cry out, or somehow amplifying the spirit.

: : I believe that Sauron had a real problem when it came to being aware of the ring. It appears that the nazgul were more attuned to the presence of the ring than Sauron was. This would make sense since the ring contained much of Sauron's spirit. Would the spirit within himself overshadow the spirit within the ring. How would he be able to differentiate between ring and himself. How would he be able to tell how far or how near the ring was. Interesting questions, to say the least. The nazgul, on the other hand, would be aware of Sauron's spirit within the ring. Being out of Sauron's presence would heighten their awareness.

: In my understanding of Sauron's relationship with The One Ring, Sauron only could command/use his spirit investment into The One, while in ownership of it. Since The One was in 'unclaimed' ownership of Bilbo, Sauron could not have known that it was near. Sauron could only connect to his realm (the one created with the rings) using his ring, the whole while Bilbo was in the realm of Sauron, instead of Sauron himself... this entirely scrapped Saurons plans with The One. In short, Sauron needed to be in his realm to see what was in it (eg. the Nazgul, the ringbearer...).

: Maedhros

Sauron was obviously able to contol the Nazgul while he did not have the Ring. Therefore he did not need to have it in order to see or sense what was in his "realm" as opposed to the real world.

Theory: Since Bilbo did not attempt to use the Ring (invisibility being a result of wearing, not using, the ring) Sauron was not aware of it any more than you would be aware of a car around the block unless it was running. The nazgul were able to track the ring at other times because they were actually looking for the car.