[Inziladun]

In ROTK, Frodo puts on the Ring at the Sammath Naur, and claims it for his own.

Immediately Sauron is aware of him and knows exactly where he is. Somehow he feels / hears the claim (the Ring is a "telephone" of sorts; possessing his own will and part of his being it is always connected to Sauron).

Now, it could possibly be said that Gollum had claimed the Ring long before, calling it "mine" and "My Precious", and so forth. Gollum had also worn the Ring very many times during his keeping of it, while he was under the Misty Mountains and Sauron the Necromancer was gathering his strength in Mirkwood just across the Anduin.

So, why wasn't Sauron aware of Gollum the way he was later aware of Frodo? Yes, the Ring was at the apex of its power at Mt. Doom, but still: how did Sauron miss Gollum having it?