I've had a few customers tell me that they didn't receive email statements. Do you have any way of checking on this?
In order to review the sent statement results you can navigate to the customer statements screen (Account > Accounts Receivable > Customer Statements). Enter the date the statements went out and click on Sent Results button on the bottom left corner of your screen. This will show you which statements went out successfully and which statements failed. If you would like to resend your customer statements for an individual or a group of customers follow the instructions in Resend customer statement.