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In Harry Potter and the Deathly Hallows, we find out when Ron returns to Harry and Hermione that Lord Voldemort put a taboo on his alias.
We see that this works in the chapter "A Place To Hide" when two Death Eaters find the trio near Tottenham Court Road after Harry says "Voldemort," so this taboo is already in place and working by later on in the same chapter.
After some debate, the crew decide to hide at Grimmauld Place; we know that there are certain enchantments to protect it against Snape already. Once inside, Harry's scar begins to hurt because of Voldy's anger, and Hermione says his name: "Otherwise Voldemort can plant false images in your head, remember." But no Death Eaters find them in Grimmauld Place.
Why is this? Have I missed something or did JKR make a mistake?