Thanks for reaching out to the Community about this, LauraASH.
If you have any other users set up, it's possible they could be sending invoice reminders manually. This is done by finding the transaction and using your ▼ drop-down arrow next to Receive payment to select Send reminder. You'll definitely want to check with any user who has access to this part of your books.
In the event your employees aren't sending them, or you're the only user, you'll want to use the article my colleague provided to contact our Customer Care Team. They'll be able to pull up your account in a secure environment, conduct further research, and create an investigation ticket if necessary.
I've included a resource about managing users that might come in handy moving forward: User types & user permissions
If there's any questions, I'm just a post away. Have a fantastic day!