this is my own view that when isildur claimed the ring for his own the dark lord could not do anything to regain it back as he was defeated by the last alliance and his power had greatly diminished and the nazgul had fled
as for bilbo he was far away in the shire and not even the dark lords eye could see that far away
and frodo calimed the ring to be his own in the very heart of the dark lords realm as if challenging him and at that instant the dark lord had both the power and closeness(his eye could see in the very heart of his realm) so the dark lord was aware.
and gollum was actually a very small creature hiding in deeep underground hills where the dark lords eye could not see so only frodo was chased
hope this satisfies you.