Why didn't Sauron feel the Ring?

Sauron could only sense the ring when its power was being used i.e. when it was worn. This was how the ringwraiths were able to strike Frodo on Weathertop when he put the ring on. As is evidenced by the reaction of his minions when Sauron realized the ring was at Mt. Doom, Sauron's attention had hitherto been bent on controlling events to the north and his thoughts were more than likely occupied with the possibilty that Aragorn (who had challenged him in the palantir) or possibly Gandalf himself who had risen from the dead and brought about the defeat of Saruman already wielded its power.

I would also venture to say that the wearer's will also had some part to play in hiding it's presence from the Dark Lord if he so desired it although this would become increasingly difficult as ring drew closer to Sauron and its will to make itself known to its master became harder to resist.