I would imagine that prophecy is transcendental in nature, very difficult to put into words, if not impossible.  I always imagined that she is not keeping anything from Thingol, but has given him all the information that she has at her disposal, cryptic as it might be.  Also, she is incarnate now, much like Gandalf and Saruman later on, thus her powers would be deminished, I would think.

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Then Morgoth hurled aloft Grond, the Hammer of the Underworld, and swung it down like a bolt of thunder.  But Fingolfin sprang aside, and Grond rent a mighty pit in the earth, whence smoke and fire darted.  Many times Morgoth essayed to smite him, and each time Fingolfin leaped away, as a lightning shoots from under a dark cloud; and he wounded Morgoth with seven wounds, and seven times Morgoth gave a cry of anguish, whereat th hosts of Angband fell upon their faces in dismay, and cries echoed in the Northlands.

                       -- Of the Ruin of Beleriand and the Fall of Fingolfin
                          The Silmarillion