It's not a problem I personally have ever had, but I've come across it often enough to know it's a real issue for some people. Short stories never finish, novels start spreading into two, three, four book series. What I think happens is one of two things: the writer gets event happy or falls in love with their world.
First things first: what do I mean by 'event happy'?
Let's suppose I'm being chased by a couple of baddies. I run into a deserted house, they run after me. I run to the roof, and manage to jump across from one house to another. I've got away - hooray! Oh no, there are some more baddies, so I start running again, down the stairs and into the street. I think I've lost them but - oh no, some more baddies. Off I go again, run run run, into a church, baddies close behind. I hide in the crypt, they go away, then I come out and into the street, when - oops, the baddies spot me and off I go again...
OK, that's essentially ONE event - Sarah gets chased by various baddies. Nothing has changed for me except the location where I'm running. Because nothing changes, the scene stays the same. This is although there is lots of action, lots of events. Nothing changes, so the story doesn't actually move on.
You see the same effect in some action films, for example, Quantum of Solace, the last James Bond film I saw, which was one darn explosion or car chase after another, with little meaning behind any of it. The second Matrix film was another example - lots of CGI, but no real point to any of it.
I've seen it in (unpublished) historical novels where our main character sits through various tea drinking meetings and social soirees, perpetually commenting on what people are wearing, drinking, saying, without the story moving on at all. I saw it in 'The Shakespeare Secret' which, despite relentless activity, never moved the characters on at all. And it turns up a lot in fantasy novels - recently an acquaintance complained of having to critique a fantasy novel which was nudging the quarter of a million word mark, and yet had nothing to say.
The solution? Change! Make things change for the main character on some deep down emotional level. Give them a choice to make that has serious implications. And finally, decide what the point is of all these words. That should give you an idea of where the ending is. For example, if the theme is 'Love Conquers All,' then go out there and make it happen.
I'll talk about world creation tomorrow.