Thank you for posting here. Let me help you in sending the invoices.
When sending invoices by email, you'll want to make sure to enter the right and active email address. Open the invoice, edit the Customer email field, then tap the Save and send.
Also, you'll want to advise them to check if there's an issue with their email server. This could be the reason why they didn't receive the invoices.
In addition, you can perform some basic browser troubleshooting steps. You'll want to use an incognito window and resend the invoice from there. Incognito won't save your browsing history which can result in an error. These are some of the shortcut keys: