Ask yourself what your characters really care about. Who do they want to become at the end of the book, and what will help them get there?
If you’re writing fantasy, perhaps you can add in more, smaller tests of your main characters skill. Do you have a minor villain who can make a few appearances? Does your main character need a rival?
If you don't have enough conflict because you've cut right to the chase, your character is probably tested under high stakes, and probably succeeds. Ask yourself how often you've let them fail. Add in some scenes where they make mistakes. These don't always have to be low-stakes, learning situations, or even fights with a rival where embarrassment is the only consequence. Major, costly failures for our major characters really keep the audience on their toes. They'll push your main character toward becoming who she needs to become
Remember, perfection may be fun to write about, but it’d dreadfully boring to read about. If you don't have the best opportunities in your book to let your characters fail, you can probably find ways to present them with difficult choices. How often do they say or do the right thing? How often is the right thing obvious?
How are their relationships? Does everyone get along all the time? Now, adding in some fights with loved ones can be a risky move if it's only done for "extra conflict." If you're going to add in a fight, make sure there's a good reason behind the fight, and not something that can be resolved on the next page. You can bring out deeper conflicts in character's relationships when they argue. Perhaps with each argument, a little more is revealed, and by the end of the book, resolved. Just don't throw one in unless you mean it.
Basically, the best way to "add" conflict is to check in on your "too perfect" characters. While they may not be actual "Mary Sue" types, some aspects of their storylines might be going to smoothly. It's easy to get caught up in vicarious writing: you never think of the perfect thing to say in the moment, so you let your characters say it when they're faced with a similar situation. Check in with those moments and make sure they're realistic. Humans are flawed. Let your characters own their flaws too.