[Rumil]

Is it possible that fire and water interfered with the vision (or perception) of the ringwraiths?

Assuming their vision was similar to that of Frodo or Sam when wearing the one ring, everything was quite misty or blurry except powerful objects and individuals (Glorfindel and The Ring).

They also appear to be less powerful the further they go from Sauron.

Perhaps at Weathertop they were a bit worried that Gandalf might come back and zap them if they hung around too long so relied on the Morgul dagger doing its dirty work.