The "was" es, all the "was"es. An editor's number one sign that maybe a book isn't as polished as it needs to be. I've found anywhere from one thousand to three thousand "was"es and other iterations of the "to-be" construction in novels before (averaging close to six or seven times a page in some cases).
But why are "was"es bad? Well, they aren't always bad, but in ninety percent of the cases I've found, they're indicative of passive voice in writing. Was, were, had been, is, are, to be... all of them tell instead of showing.
When appearing in the narrative, these sentences with "was" (or any other to-be) should be rewritten to be more active, which will help engage the interest of the reader and more deeply engross them in the story. Passive writing is a really bad idea for many genres of fiction (as opposed to technical books about making cheese, or blog posts about passive voice) because the reader wants to feel like they are actively moving through the story, rather than being passively told about the story.
The leads to another issue with passive voice. Passive writing hides who is performing the action, or makes the action stand alone (and this sometimes doesn’t make sense). So instead of saying “the dog was bitten,” say “the monkey bit the dog.” This not only removes the passive voice, but also gives the reader more information in an active fashion.
Therefore, in descriptions, the use of “was” and “were” indicates “telling” instead of “showing.” Look at each instance of “was” and consider what information it is telling instead of showing. When telling the reader some things about the character is good, consider how this could be shown to let the reader experience it more fully. For example, "his hair was blonde" tells us information about the character, but "his blond hair fell in front of his brown eyes as he looked down at the lab experiment twitching on the specimen table" gives us significantly more information and ties in more components of setting and plot.
Passive voice is especially harmful in action scenes since it makes the action happen more slowly and less immediately. Examine every sentence and make sure that you’re showing the reader what the character is doing in real time, not telling them in replay. A common offender of this is the combination of “was” + “-ing,” such as “was running.” This word can easily be replaced with the root verb to instantly make it active. Sometimes the was+ing is necessary, but ninety percent of the time, it makes the sentence unnecessarily passive. Also with passive voice and telling, it often doesn’t show who is enacting the action. “She was dragged away” doesn’t tell us by whom. “The guards dragged her away” is active and tells us who is doing the dragging.
Go through every sentence and see whether it is telling or showing what is going on. By this I mean: is the reader being told what is going on (this happened) or is the writer showing them and letting them experience what is going on (so and so did this)?
If you want to craft good fiction, first drop the "was"es and instead focus on making the characters and plot come alive for your reader. Show, don't tell.
For more information on passive voice, please refer to this resource: https://owl.english.purdue.edu/owl/resource/539/3/