In Gmail, why are my replies to emails not showing up in the original thread?

Question

I have several Gmail accounts. Usually, when I reply to an email I receive, my email is viewable in the original thread it was sent in. However, I recently made an additional Gmail account through my college's portal, and when I reply to an email, or when someone else replies to my email, it comes up as a new thread. This is wildly irritating. Does anyone know how to fix it?