Discount codes not working when accessed via email marketing

[QTS]

Hi all,

I have recently discovered that when I send a discount code in a Mail Chimp created email and my customers access my Shopify website using the email links that their codes don't work. They have to load the website in a new, fresh window. This happened 3 times so far. 

Is there any way to resolve it? 

Thank you for any advice on this.

Romana