Question: I am writing a story, I think it is flowing well. But, it seems to be so short compared to books I read and my plot seems to be developed. but in a short space. How can I increase the length of my book, whilst retaining my plot?Answer
I'm not a big believer in padding. Generally, you should write the words you need to tell your story completely, and no more. A book that contains a lot of unnecessary verbage is one that needs a good editor.
Generally, a work can be considered a novel if it is over 40,000 words. If you're over that, relax.
On the other hand, sometimes it's okay to write a novella rather than a novel. Certainly, the ebook format makes it less of a problem to publish a shorter work.
True, you can turn a short story into a novel length work by exploring certain areas in more depth, going into more detail with subplots etc. But it's not always an improvement. Personally, I enjoyed the short story "Flowers for Algernon" much better than the expanded novel-length version that came later.
Another option, if you have some minor characters who interest you, is to write two or three novellas, each focusing on a different character's story. The result is more like an anthology than a novel, except that the stories will be connected because some of the same characters show up in more than one story.
An example of this is Stephen Leacock's Sunshine Sketches of a Small Town
which is an anthology of short stories all set in a particular community in Ontario, Canada.
Or consider Ursula K. Le Guin's book Always Coming Home
which is an anthology of stories, poems, plays, a novella, etc. What unites them is that they all take place in the same fictional community in the far future.
Another option might be to write stories about different generations of the same family, set in different time periods.
Whichever option you choose, make sure you are writing material that matters, that inspires you, and that makes the work better. Don't just add material because you want to make the book longer.