It is a quite common problem that a recipient hasn't responded to an email that needs a response. For people who send a lot of emails, it can be quite a complex task to see which ones have been responded to and which one not ... and follow up on latter if needed.
One method I have found particularly useful in dealing with this is adding my email address in the Cc line if I this message requires a response. By doing this, I receive a copy of the message which I then put in a folder I call „Follow up“.
Daily I just check in this folder, and delete the ones that have been responded to or follow up on the ones that I think are overdue.
Pretty simple but effective way.