Your main character’s story problem should be a big enough problem to carry them through the entire story and not be resolved until the climax. If the story problem resolves in the middle, it’s not a strong enough problem.
For romance, you also want strong conflict between the hero and heroine so they can’t get together during the story without some serious consequences. There has to be some type of relational conflict keeping them apart.
For non-romance, there should be strong conflict between the protagonist and an antagonist. Again, it should be strong enough to keep them from resolving their differences halfway through your story. They should be pitted against each other—with good reason to fight each other—all through the book, not resolving their differences until the climax.
The story problem should be deep and personal. Beyond external events, the characters have deep motivations that drive them to fight each other.
My friend Janet Dean quoted me this, which she got from bestselling author Susan Wiggs:
Think of your heroine (or protagonist), then think of her worst nightmare and make that the hero (or antagonist).
In Bayou Justice by Robin Caroll, the heroine, CoCo, is a strong woman who works fearlessly in the Louisiana bayou. But her greatest fear is being hurt again by the man she loved, Luc Trahan. The story problem is that she’s a suspect in the murder of Beau Trahan, who had been about to evict her family from their home, and who happens to be Luc’s grandfather—thus throwing her together with the man who could rip her heart out again.
In When Dreams Come True by Margaret Daley, the heroine, Zoey, fears a disruption of her life just as she and her children have settled into Sweetwater. The story problem is that her husband, who had been believed dead and who had been estranged from her before his disappearance, shows up on her doorstep and needs to be reassimilated into his own family.
In Sushi for One?, my heroine fears a relationship with a man and makes a stringent list from Ephesians to weed out undesirables—a type of personal coping mechanism. The story problem is that she keeps being thrown together with Aiden, who doesn’t meet any of the requirements on her list.
So think about your protagonist and antagonist. How can you tweak things so that the story problem becomes deeper and more personal?