David Heyman, the producer of the movies, was speaking to the American magazine Entertainment Weekly.
He said: "With the first two books there was no pressure. By the third book there was a little pressure. And with the fourth book, she faced huge pressure.
"Right now, Jo is just taking the time to write the best book that she can."
Part of the problem is the time it will take to get the book ready and printed - probably five months once JK has finished the book.
So, even though publishers Bloomsbury said in May that it might be out by the end of 2002, it's now almost impossible we'll get our hands on it this year.
Thanks to Leaky Cauldron for the info.