It may be interesting to note that the nature of Amon Hen may have amplified the effect when Frodo put on the Ring. The Seat of Seeing allowed people to see the surrounding lands as if through a telescope and may have accidently 'amplified' the 'I am here' beacon the Ringbearer would have on Sauron. This would make sense since Sauron detected neither Gollum during his many long years under the Misty Mountains, nor Bilbo during his journey through Mirkwood, in particularly during his battle with the spiders during which Bilbo used the Ring extensively while Sauron resided in Dol Guldur.
EDIT: Ah, bugger. I see Raynor already made the same observation about Amon Hen. That's what you get when you hit 'Reply' too soon. 
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'I am the Elder king: Melkor, first and mightiest of all the Valar, who was before the world, and made it. The shadow of my purpose lies upon Arda, and all that is in it bends slowly and surely to my will.'
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