What could cause this to happen is the member removing their email address on file (such as deleting their email from online banking). This email required for a member to receive an eStatement.
When the email is removed the system will send a paper statement. When the email is re-added, this will trigger the eStatements again.
To check for account maintenance you can use Tool #159 Audit CU File Maintenance (CUFMAINT).
See additional information below.