Apart from what a story is about, there is also the matter of how it is written. Once the reader is caught up in the story they can find themselves being drawn irresistibly through the exciting parts but just as irresistibly through slower sections. To some degree this is to do with being engaged with the story overall and the characters, but it is also to do with the way it is written. The way the words are put together, the construction of the narrative, the syntax used to create a rhythm.
Some people are obviously just gifted in this regard. They naturally put words together in an attractive way. But that doesn't mean it can't be learnt. On the most basic level there is spelling and grammar and typos. Just being able to read the text will make the flow better. But assuming those things are at a competent level, there is another level of sophistication beyond that, which will make the narrative flow.
To make the prose read with greater fluidity and momentum what you need to do is vary the length of sentences and paragraphs. This may sound simplistic but it has quite a marked effect on the reader.
If you have lots of sentences that are very short the effect is to make it read stilted and childish. If you have too many sentences that are too long it becomes convoluted and rambling. By mixing things up it creates a rolling momentum that keeps the brain from stalling out of the story. Similarly if every paragraph is about the same length it becomes easier for the brain to start tuning out.
The length of sentences and paragraphs has an effect, one that is easily achievable by anyone with a rudimentary knowledge of language, but you then have to be able to choose when to employ these effects — you have to have a purpose.
The importance of what you're saying to the story as a whole is related to how much space you give it. Although most people will naturally give a subject the amount of space that seems appropriate, you can manipulate this understanding by exaggerating. Extending the length of the piece for emphasis, or shortening it to undercut its importance. But once you decide how important the subject is (to the sentence, to the paragraph, to the overall narrative)you can then decide how many words you want to attribute to it.
There's also a relationship between the amount of time that passes within the story and the length of narrative you use to convey that time. This doesn’t necessarily have to be a linear relationship. For example if I say: Six years later he returned home — then the way I’ve contracted six years to six words will have an effect on the reader. Similarly if I say: The cup fell from her hand, spinning twice before smashing to pieces, large and small, around her feet and all the way to the door – here I stretch a couple of seconds so that it feels like time has slowed own.
The trickiest part is writing the longer sentences. Most people lack confidence with compound clauses and the like, or go overboard and let things get too convoluted. The best way to resolve this is to read it out loud to yourself. It’s a little tedious but what it sounds like is a very useful guide to how much sense it makes.
Basic as this sounds it really does make a difference.
1. Mix up the lengths of sentences (and paragraphs).
2. Choose which sentences (and paragraphs) to make longer or shorter based on the subject and its importance to the story overall.