Openings come easily to me. As do endings. I always know where my book will begin and where it's going. Things can change, of course. Nothing's ever set in stone, until it's published that is (and even then, some authors create 'difinitive editions of their work, reediting long after it's been read by multitudes of people).
For me though, the middle is hard. I know where I want my characters to go, I've got a generalized outline of my novel, but somehow, in the middle, things get bogged down. That's where I am right now in the fantasy novel I'm writing. I've got the first several chapters written and the ending's pretty clear in my mind. There's just ALL THE STUFF IN THE MIDDLE to go. And it's not going.
This happened in the last fantasy novel I wrote. That time I got over it by simply plowing ahead, writing whatever seemed to fit and editing it later so that it really DID fit. This time... I took a break by writing another novel in between, something completely different (ie not fantasy) that took about a year and 'cleared my mind'. Cleared it a little too much, I fear. Now I'm back to working on it and try as I might it just isn't coming. I've tried forcing myself to sit at my computer and type - until something else 'important' comes up and I have to go do THAT instead.
One of the authors I interviewed for my other blog (www.scififanletter.blogspot.com) mentioned that the middle often causes authors to doubt. They doubt themselves. They doubt their story. They doubt that anyone will publish it someday. Apparently it's the middle that causes a lot of wannabe writers to give up, deciding they just don't 'have what it takes'.
I guess I am lucky in that I know I 'have what it takes'. I've finished two novels. Nothing's published yet, but I'm comfident that's only a matter of time. I'm planning to have this new book done in time for Book Expo America next year so I can shop them around to the agents in attendence.
That requires getting past this 'mid book block' and completing the book. Which requires determination and dedication. Ah, it's easier to READ a book than write one. Though not half as satisfying!