The flow of your language usage is something that isn’t always readily apparent as you’re just writing. You go along, mindful of usage and what you’re trying to say, but prose writers aren’t always concerned with the poetry of language or even simple ease of comprehension. If you know what you’re trying to say, of course your reader will too, right?
Whenever I’m tasked with reading someone else’s fiction for feedback as an editor or beta reader, I know too often I use the cryptic note “awkward” without much explanation. And yet sometimes it’s just that…it’s awkward, and I can’t always articulate why. One cure I’ve found helpful occasionally is to force someone to read these awkward phrases aloud. This is often just enough to demonstrate that intangible problem with the flow of the language.
I think, too, reading dialogue aloud in particular can be quite helpful, because it can help pinpoint things that don’t sound natural or appropriate to the era your story is set in.
Long story short: if it sounds like the person reading the audiobook version of your story would stumble over it, it should be rephrased.