Before I begin, a caveat. This blog is and always has been directed towards commercial fiction. However, that doesn't mean the literary fiction writers should quickly hit the Back button. Hold on and hear me out. If you take my advice with a certain amount of salt, it can still be very helpful to you. After all, the greatest author of English literature in the past half-millennium was a commercial fiction author. (Those that disagree with that statement are usually those that discount Shakespeare simply because he was a commercial fiction author.)
Ok, that's over with. On to my point.
How do you answer the question, "So, what's your book about?" Do you hum and haw about theme, or else dive into the MC's entire backstory? Do you spend fifteen minutes giving a blow-by-blow of "what happens"? Do you simply give a one-line explanation of the premise... and stop there?
Well, my friends, these are all examples of a lack of plot. This doesn't mean things don't happen in your book. I'm sure they do. What I mean really is that your plot is either ill-defined or swallowed up. So, we'll look at all four of these examples and then see how to solve them.
Thematic Blockage (you hum and haw about theme)
If you spend all your time talking about your novel's theme and message, then it's a clear sign that you have fallen into the common literary trap (and I do mean literary) of thematic blockage. What I mean by this is that you have been focusing on the message of the book and not on, well, the book. Your book may have a lot to say, but you'll have to make us sit down and listen first. You do that by entertaining us. Mark Twain once said that a classic is "a book which people praise but don't read". You need to find your plot.
The classic rules of plot writing apply here. You need a protagonist (doesn't need to be a "hero") who has a problem to face. There should be one or more obstacles to this goal. The novel is about overcoming these obstacles in an attempt (whether successful or not) to reach this goal. There are tons upon tons of resources on this subject, so I'll spare you the details. All I'm saying is, get a plot. Your message will be better for it.
Character Assessment (you dive into the MC's entire backstory)
If you find yourself talking more about your character than what happens, then you also have a plot problem. Sure, you have a great, well-crafted, and interesting MC. However, it isn't a novel until they do something. And I don't mean the dishes. Again, we need the basics. MC (check), goal, and obstacles. This is especially important in character-centric stories. Authors of these types of stories say, "Well, my book isn't really plot-driven. It's character-driven."
Not so, I'm afraid. All that classic phrase should mean is that your goal is an internal one. Is your book about a conflicted Nazi in World War II? Well, the goal is the character's "salvation" and the obstacles are his job, his familial expectations, the Nazi propaganda, and perhaps even his own pre-existing prejudices. Looks like MC, goal, and obstacles to me. Now start crafting your plot around these details.
Drowning Plots (you give a blow-by-blow of "what happens")
Another cause of seemingly plot-less books (this was my problem) is actually an excess of plot. Perhaps you have a lot of stories to tell. Perhaps you just love watching your characters interact, and you have an excess of fat. Perhaps you haven't quite committed to a specific MC yet at all. Whatever the exact reason, the result is a story that is so bound up in events that you fail to have a clear plot.
Go back to the formula we've been discussing. Who's your MC? If you don't know by now, decide. Today. Now, what's the goal? Not sure? Well then, figure out what you're building towards. If you don't have any clue where you're going, how do you expect to ever get there? Decide where the book ends. Then, you can determine what the goal is, because that will tell you where you should be working towards. Then, figure out what are actual obstacles to that goal. You can only have ONE goal. One. Just one. Only one. One.
No, you're book isn't different or special. Stop arguing with me. One.
So, once you have that, all else is fat. Back to the trimming the fat discussion. If it doesn't work towards that goal, it can go. To mix metaphors, cut out the weeds that are chocking your plot.
A Premise Problem (you give a one-line explanation of the premise and stop there)
There is NO SUCH THING as a premise-driven plot! A premise-driven plot is like a sports car with no wheels. I don't care how cool your premise is, if you don't have a plot to back it up, the premise is irrelevant. Trust me. I know. I've fallen prey to this myself.
Let's say you have a good premise for a book plot, perhaps... * cough* ... your book is about a girl who goes to school to become a magical spy. Well, that's great, but unless your (that is to say, my) MC doesn't have a particular and identifiable plot, then the premise isn't going to get me anywhere. What I had to do was find out what her goal for that book was and work the plot towards it. I knew where I was going towards, but I didn't do anything to tell that to the readers at all. When I introduced the first breadcrumb, the reader didn't know. I had the excess of plot problem we mentioned before, and how were they to know this wasn't yet another subplot? Readers will only go with you so far. I had to find a way to tell them that this was important without telling them everything that was going on. So, I wrote a scene that made the importance clear, but kept the mystery going.
Like I've been saying, my book needed a MC, a goal, and obstacles. I really had all of them, but for my premise to translate into a plot, I had to identify them and craft the novel so that it worked with them and towards them.
And that's what you need to do too.