Customer only receiving 1 email for 2 separate sales

Hello, I have many customers email me to advise they have only received one order confirmation email (the 1st sale) but they have placed multiple separate orders. I can not see why it would do this on our website as everything is set up and working correctly apart from when a customer orders products in separate orders.

Please can you advise if I need to change a setting?

Love your work and stay safe