Cart Not Clearing After Checkout Complete

I have a customer who orders daily and occasionally they said their cart doesn't clear the contents after successfully placing the order. I have tried to duplicate it but can't. Has anyone experienced this or have some suggestions of where to look? One thing different than most of our other customers is they use an offline payment option (purchase order) at checkout.

The customer sent me an image today showing that they checked out successfully but the items are still in their cart - attached to post.


Image didn't attach: https://www.piecesofhistory.com/cart-not-cleared.png

It can depend on payment method. What payment system do you use?

It can depend on payment method. What payment system do you use?

This customer is using a "purchase order" to complete checkout and no payment info is collected. This payment method uses "Processor: Offline" and "Template: None" in the the payment method details. What makes it difficult is I can't find a pattern and it's not every order they place. It appears to be random.

Thank you for the clarification. It looks like sessions issue. Please contact CS-Cart support team to examine the problem on your server