Why did the Nazgul not succeed in Bree and on Weathertop?

[The Saucepan Man]

Great material there from Raynor and the young Brandybuck. You learn something new every day ...

As for Weathertop, the Nazgul did at least partially succeed. The Witch-King stuck Frodo with a Morgul blade. He knew what the effects of that would be. At best (from the W-K's perspective), Frodo would succumb and become a Wraith himself. With no power to resist, he would don the Ring, slip away from his companions and easily be found by the Riders. At worst, it would slow down the companions' journey to Rivendell, possibly allowing the Riders to attack at full strength at a moment of their choosing.

Add to that the material provided by Raynor and A-Brandybuck, and it probably seemed to the Witch-King at that point that discretion was the best part of valour.

Of course, he didn't account for the fact that Glorfindel (and other Elves of Rivendell) were abroad - a foolish oversight, perhaps, but not one lacking in credibility. He underestimated the strength of Frodo's spirit - but how could have understood this? And he overlooked the healing skills of Aragorn (and the presence of athelas in the region) - something he should perhaps have had some conception of, but again not an oversight lacking in credibility.

Originally Posted by Nogrod

To my eyes they gain a lot of power between FotR and RotK. On Weathertop they drew back from a torch-wielding hobbit, in Minas Tirith the "newly-born" and mightily strengthened Gandalf has trouble dealing with them... Is this logical? I doubt that.

Tolkien states in one of his Letters that the Sauron imbued the Witch-King with added demonic force before the march on Gondor. So the Witch-King, at least, did gain power between FotR and RotK.

[Rumil]

I've also had the impression that the Nazgul became more powerful the closer they came to Sauron in Mordor. During the passage of the Dead Marshes Sam and Frodo were more terrified by the cries of the Nazgul than they had been in the Shire.

I wonder if this could be explained by the intervening rivers. Tolkien stated the the Nazgul were reluctant to cross running water, which seems peculiar, but similar stories are told of witches and sorcerors in folklore. It has been proposed that the runnning water interfered with their perception in some way (sort of messing with their vibes!), perhaps making them less sure and resolute in their actions than they would have been on 'home turf'.