So why did it not then slip off Frodo at any of the opportune moments in which a Nazgûl was near (e.g. Weathertop, Flight to the Ford, etc.)?
Gandalf gives what may be the answer:
'Behind that [Bilbo finding the ring] there was something else at work, beyond any design of the Ring-maker. I can put it no plainer than by saying that Bilbo was meant to find the Ring, and not by its maker. In which case you also were meant to have it. And that may be an encouraging thought' ("The Shadow of the Past," The Fellowship of the Ring, 81).
If Frodo was "meant to have it," then it may be he was meant to keep it by that "something else at work."
But I am curious if Tolkien addressed this in any other writings. (NOTE: I've accepted an answer that does not address this, and still hope if some other writings exist discussing this, that they will come to light here.)