WordPress search doesn't find some posts. How to fix it?

Hi ??

I struggling with search on a blog I manage. It just doesn’t find all the post that it should.

So lets say I have posts like:
1. “Motivation”
2. “Confidence”
3. “Thanks a lot!”

If I search eg. “confidence” everything seem to work fine. But for some other keywords (eg. “motivation”) it just don’t find all the posts that have word “motivation” in the title and I have no idea why.

Have you faced this problem before and have any clues how to deal with it?

Stachu