The Real Reason the Nazgul Left Frodo Alone
Phil L.

The answers already submitted in response to the question of why Frodo was left with the ring on Weathertop certainly are good points. But I think the overall answer was their arrogance. Their model, Sauron, never even for a second considered anything other than complete success. Since not all the Nazgul were there that night, and since Aragorn was there (plus they did leave the knife in him) they merely decided to wait for the rest of their crew and attack together. Patience is something that Sauron, much more than Elrond, Gandalf et al, seems so much better at - except when it really matters at the end...