It very unlikely your customer did not receive an email, But it is possible in rare cases.
Reasons Your customer did not receive email.
Email is in their spam folder
Their email service provider blocked the email.
Customer Missed the email, never Seen it.
Solutions:
Copy the Revision link from inside the Ecomartists app, and send to customer from your store email.
Leave feedback on behalf of your customer
Approve the drawing on behalf of your customer
Do nothing, We will automatically send another email every 3days, if customer does not approve or leave feedback .