Have you checked your email spam filtering?

In my own store it is not rare for a customer to email me their registration details,
me to reply back within a few minutes with their registration file,
then a couple of days later receive an angry email demanding that I fulfill my duty and send them their registration.

I have no way of knowing that their email client, or ISP, or dog is eating my emails.

Check your spam filtering.

Try sending via a different email address. When this happens to me I use my alternative Yahoo account. It is less likely that my primary AND secondary emails get blocked.

Hope that helps.