[Firefoot]

"The Ring now has passed beyond my help, or the help of any of the Company that set out from Rivendell. Very nearly it was revealed to the enemy, but it escaped. I had some part in that: for I sat in a high place, and I strove with the Dark Tower; and the Shadow passed. Then I was weary, very weary; and I walked long in dark thought."

-Gandalf, in The Two Towers, Chapter V, The White Rider

Another thing is that at about the time Frodo and Sam entered Mordor that Sauron was waging war against Minas Tirith, Lothlorien, and the Dwarves of the Lonely Mountain. His efforts were concentrated there; and he thought that someone in Gondor had the Ring. He was not looking to his own lands.

"Wise fool. For if he had used all his power to guard Mordor, so that none could enter, and bent all his guile to the hunting of the Ring, then indeed hope would have faded: neither the Ring nor bearer could long have eluded him. But now his eye gazes abroad rather than near at home; and mostly he looks towards Minas Tirith. Very soon now his strength will fall upon it like a storm."

- Gandalf, in The Two Towers, Chapter V, The White Rider

His doubt will be growing, even as we speak here. His Eye is now straining towards us, blind almost to all else that is moving.

-Gandalf, in The Return of the King, Chapter IX, The Last Debate