- fails at basic hygiene (solved by showers, haircuts, clothes, cleaning your room, home improvement projects, eating vegetables, etc)
- fails at basic rapport (solved by practicing with non-romantic partners)
- underlying mental health issues (solved by your therapist/psychiatrist)
- fails at having an appealing life to join (solved by identifying personal goals and practicing working towards them)
There are exceptions, of people who are extremely physically unattractive, or who have truly insoluble mental health issues. But it's a really tiny percentage of people. I'm talking you literally make babies cry with your appearance, or you have a history of torturing animals, or something like that.
Most people have just a couple of actual dealbreaking items on their filter, so out of 20 candidates there should be one or two whose filters are on things you accidentally do right.
And I'm not saying those four things I listed above are quick to solve. They take time. It might take 2 years of daily effort. I'm just saying they're easy to solve: there's no trick. Anyone who can bake a cake can do the steps necessary to solve them.