The words are not flowing

Wow, I've had a really difficult couple of weeks on my current novel. The words are not flowing as freely as they were in the first three books in the series, and on more than one occasion I've been tempted to leave the book and go on to something else.
    The strangest thing is the main characters in this novel both appeared in previous books and they happen to be my favourite characters in the series. Because of this I can't understand why it's so hard for me to tell their story. Before I ...