Search Couldn't Find Several Pages

In March I’d taken some notes and created around 8 or more pages of text. Today I tried to find text in these pages with the search function and it couldn’t find anything.

So I did a Windows Search of my project directory and it found all the pages based on a book name. I then found the pages manually in the binder, with some difficulty, and resaved them. Only then could I search for text in them.

What could have caused this? I’m not worried about losing material because everything I’ve done should still appear in the binder. However, not having 100% search functionality is a worry and can lead to some confusion.