Things need to work out in certain ways in a story, but there’s a fine line between events that read plausibly and ones that feel like a series of unlikely coincidences. Plots become strong when events happen for reasons rooted in character goals and motivations and not just because the author wrote it that way.
Because I Said So
You can’t always cut them out completely, and at some point in your story there’s bound to be a coincidence. Two people visit the same spot at the same time, someone walks away at the right moment, an item lost shows up at the worst possible time. Stories happen when people and events converge, so naturally the book will occur around those convergences. It becomes troublesome when a high percentage of those events rely on coincidence to make them happen, because that stretches credibility for the reader. When they stop buying it, they stop reading it.
The coincidences to worry about are the ones that plot hinges on. Your protag needs to find X and X shows up exactly when they need it. The person your protag needs to meet just happens to be at the restaurant where they stop for lunch. Your protag is always in the right spot to overhear critical information. The things where the protag does nothing to bring about the desired outcome but be there.
And that’s key. The protag needs to act and make plot happen. When things fall in their laps, the sense of wining is gone and the story feels stale. It’s no longer about seeing someone struggle for victory, it’s watching them get handed that victory.
Look at the important plot moments in your story. The ones that couldn’t be taken out without the story falling apart.
Does the protag act in a way that causes this event to happen?
This can also be the result of something they did, such as a previous action that resulted in this consequence. But your protag did more than just show up at the right place at the right time. Their goal led directly (or indirectly) to this event happening.
Does this event complicate the protag’s goal in some way?
While bad things happen all the time, random bad things in a novel usually don’t work all that well. Just making it harder seldom makes it a more compelling problem, and it can even verge on melodrama if you take it too far. The event or complication should relate to the protag’s goal, or a choice they made. The complication is a result of an action. They chose to ignore A to deal with B and now A is coming back to bite them in the butt. Or they tried to fix X and that made B happen.
Do the other characters in the story, especially the bad guys, have a plan?
Antagonists with plans and goals of their own make much better bad guys, even if you never get inside their heads or see them on screen. But their actions have meaning and that keeps them from feeling random. Their plan is grounded in strong motivations and goals just like your protag, so even when the protag is trying to solve one problem, the antag is chugging along on their own causing trouble.
Wanting it isn’t always enough.
If your character is always after something for a reason, you reduce your coincidence level considerably. But they also have to work for it. They have to uncover clues, overcome obstacles, face internal struggles, do the things that make figuring out the solution plausible.
The more they work for it, the more they’ll earn it, and the more believable the outcome will be for your reader.
And that’s no coincidence.