We see that the Ringwraiths aren't affective. They fail to get Frodo while he's in the Shire, they fail to get Frodo while he's on Weathertop, they fail to get Frodo wondering around in Mordor. We also see them fail against fighting Aragorn on Weathertop. Even Gandalf admits that he can not take on all nine by himself, they are tough opponents. Aragorn is able to take on five of them by himself, which suggests the Wraiths weren't at their full potential, maybe they didn't have their rings? Since they seem to not be at full "strength." This is just my own speculation though
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