To keep your readers' attention, you need to use sentences of many different lengths. Some should be long and flowing while others should be short and to the point. If every single sentence in your document is the same length (or close to the same length), your reader will be bored. If your sentence lengths aren't varied enough, consider changing things up.
We calculate sentence variety using a mathematical concept called standard deviation. It's complicated to explain, but basically, the higher your score, the more variety in your sentence lengths in your document.
Dialogue and action scenes, for instance, can benefit from shorter sentences, while descriptions of setting or mood might need to be longer and more flowery. Think about what you're trying to convey and rewrite your sentences accordingly.