One of the reason given in the book is Frodo calling on the name of Varda. The latest explanation comes from the Reader's Companion by Hammond and Schull:
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Originally Posted by LotR: Reader's Companion notes to page 208
...the Witch-King, the great captain, was actually dismayed. He had been shaken by the fire of Gandalf, and began to perceive that the mission on which Sauron had sent him was one of great peril to himself both by the way and on his return to his Master (if unsuccessful); and he had been doing ill, so far achieving nothing save rousing the power of the Wise and directing them to the Ring. But above all the timid and terrified Bearer had resisted him, had dared to strike at him with an enchanted sword made by his own enemies long ago for his destruction. Narrowly it had missed him. How he had come by it – save in the Barrows of Cardolan. Then he was in some way mightier than the Barrow-wight; and he called on Elbereth, a name of terror to the Nazgűl. He was then in league with the High Elves of the Havens.
Escaping from a wound that would have been as deadly to him as the Mordor-knife to Frodo (as was proved in the end), he withdrew and hid for a while, out of doubt and fear both of Aragorn and especially of Frodo. But fear of Sauron…. was the stronger.
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This is supported by statements in Sauron Defeated (HoME VI I believe) about the value of the swords, refferences which didn't make it in the final version.