Question: Someone in a class I'm taking said my mystery needs to be more plot-driven. How can I tell if she's right and if she is, how do I fix it?
Answer: Well there are two kinds of mystery. Plot-Driven means it's all about what happens and the investigating character does not change. Most mysteries used to always be this way. Character-driven mystery is about characters that change and grow over the story, or even over the series.
The plot-driven mystery is all about the puzzle. Sherlock Holmes is always his superior, calculating, self. At least he is until he meets "The Woman." And even afterward any emotion or resulting change in his personality is never part of a Sherlock Holmes story. Yes, it is important to have a good puzzle for the reader to solve. And your reader might be telling you that the puzzle might be a tad transparent. If that seems reasonable to you, lay some more false trails, leave some false clues to lead the reader to a false conclusion or two.
Plot-drive mysteries are all about the plot and only about the plot. Think about it. What do we know of Hercule Poirot except that he is Belgian and fond of his "little gray cells?" Miss Marple is elderly, a bit cynical on the subject of human nature, and knits. Their presence as characters is only to facilitate an answer to the puzzle. We rarely know what they're thinking, which way they'd vote in an election, or what issues are important to them. Then there's the hard-boiled Dashell Hammett, Mike Shayne plot-drivers, where the most important thing about the characters is that they act tough, drink a lot, and follow the clues to find a solution. It's been awhile since I read any Lillian Jackson Braun(sp?), but I think maybe her "cat" mysteries fall into the plot-driven category, too. Though she might be vulnerable if she lost her cat.
Even so, in my opinion, the character-driven mystery is the most enjoyable. Readers like to read about characters they care bout. A mystery where the main character's personality, and emotional needs play a role is always preferable to me as a reader. It makes the story richer. I care more about what happens.
I've never read a really bad one. Even in Dick Francis's first, ODDS AGAINST (which had a horrible hole in the plot) the writing was terrific. His main character, Sid Halley, has lost his marriage because his wife didn't approve of his career and his career due to an accident that left him physically handicapped. He's depressed and all but suicidal when his ex-father-in-law asks him to look into a mystery. In all his mysteries the main character is vulnerable and stands to lose a lot.
But you asked about plot driven mysteries, Agatha Christie, Margery Allingham, and some would add TV's Jessica Fletcher to the list, write plot-driven mysteries. Plot drives most of the action in "puzzle" based mysteries. They are almost all about plot and the characters are chess pieces, moved about to accomplish an end. Jessica Fletcher is always Jessica. She never grows, or changes, or stands to lose anything but face, or one of MANY nephews, if she fails to find the killer.
Hope this helps.