Table of Contents not updating

I have successfully created a Table of Contents. Within the table I want to add a Level 2 title, for a paragraph I just wrote, under an established Level 1 title. But when I try to add the Level 2 title to the Table of Contents, it doesn't show up. There is already another Level 2 title that was written earlier and does show up in the Table of Contents. The level 2 title I'm trying to add would show up above the one already there. Does anyone know what is causing this not to work? Thank you in advance.