The first thing that comes to mind is do you have enough plot? There's a great article about this in the knowledge base. The basic idea is, is there enough plot and is there enough happening to support a full-length novel? Is there an internal and external plot arc? Are there subplots? Do supporting characters have arcs? Where can you add more meat to the story?
Also, I suppose I should have mentioned this first, but when you say you're missing writing goals by 20k, how much are you actually writing and how long is the finished novel? Depending on what age group and genre you write, your word count can vary a lot and there's a chance your word count is okay!
Let's say you have solid arcs and you have enough subplots and the supporting characters are all fleshed out with their own arcs, think about adding in more details. I wouldn't call myself an underwriter, but adding in description is almost always something I have to go back in and consciously add. There are a lot of exercises out there related to developing your description muscle and you could try some of that out before you try to apply it to your novel.
Finally, what are your readers saying? Has there been any consistent feedback from people who have read your story about what they feel is missing? That might also give you some direction about where you can add to your story. If you haven't had beta readers or critique partners look at it yet, I'd consider that as well and maybe a reader could find parts that you could expand upon.
It does not do to dwell on dreams and forget to live.
I go over my word count all the time. Usually it's because I make the plot too complex. The only way I know how to really compensate is by plotting as if I'm writing for half that amount of words and then hopefully I'll stay under it.
Maybe you could do the reverse? Plot your story out as if you're planning a much longer story, and then it will give you more to keep it going? I don't know if that would work, it's a total guess!
Another thing to think about is why you're setting your goals at a certain word count. If you're missing your goal by 20k words but are still writing 100k, then that's still a very sizeable piece of work!
If, on the other hand, you set your goal at 40k and miss by 20k then suddenly that's quite a big chunk you're missing.
I'd really have to agree with Carlito on this one, more often than not it comes down to your plot. I always used to underwrite (mainly because I was so keen to get on with the story that I would anxiously skip ahead to the more interesting parts).
Think about the development of your MC and other supporting characters. Where do you want them to be at the end of the novel vs the beginning? What have they learnt? What experiences can they have along the way that would help them to grow and change?
A beta reader is also a good point, as other people often find gaps you've missed. If you're looking for someone to read things, there are a lot of people on YWS who would happily volunteer (myself included)
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