On a few occasions after I have sent an invoice reminder, our old clients have complaint that they have never received the invoice in the first place. Is there a glitch?

Hello there, vitamartinenko.

It's possible that the invoices were placed in their Spam or Junk folders. We can let them further check it and if it's not there, you can clear and then re-enter your email address.

Let me show you how: 

1. Go to the Gear icon and click Account and Settings.
2. In the Company menu, click the Contact info section. 
3. Clear the Company email field and then Save. Enter the desired address again.
4. Select Save and then Done.
5. Try sending an invoice to yourself to verify if it works as expected. 

If you get the same thing, you can follow step 3 on this article: 3 solutions for when customers aren't receiving your emails.

You'll want to check out these links for more details in handling your sales forms: 

• Email a sales form or report to multiple email addresses
• Send invoices from your Gmail address FAQ

Keep me posted on how this works. Take care and have a great day!