LotR -- Book 1 - Chapter 11 - A Knife in the Dark

[Inziladun]

Originally Posted by Animalmother

Why fool around with Morgul knives and such? The ringwraiths had swords and thousands of years of sword-and-sorcery experience. Strider bore a torch and the hobbits bore small blades of Westernesse.

It does seem a bit like they weren't giving Sauron their best at that point. Then again, they also missed the Ring at Hobbiton because they took an old hobbit's word that Frodo wasn't home.

Anyway, I think at Weathertop it goes back to two factors. One, Aragorn had said the Ringwraiths feared fire, and that seems to have been proven by him. Two, as Aragorn also noted, he and the Hobbits still had a long way to go before they could have reached any possible safety, and the WK seems to have had the thought that turning Frodo into a wraith was the easy way to do things. Gandalf later said that the splinter in Frodo probably would have turned a Man fairly quickly, and the WK can't be faulted for knowing so little of Hobbit resilience.

Originally Posted by Animalmother

So, these couldn't be the same ringwraiths who made the defenders of Minas Tirith quake in fear.

I don't have Letters with me, but there's a letter in which Tolkien notes that the WK had been given an "added demonic force" when he was placed in command of the forces attacking Minas Tirith. I think all the Nazgûl were at an elevated level of power at that point.