My Miva Merchant is set up to notify me of each sale by an e-mail. About 2% of the time, no notification is sent to me, but an approved and funded sale appears on my credit card processor's listing of my orders.
We use Miva and if this is happening it it probably one of two things. 1) Your Email provider is filtering out Miva's automated emails as junk (which is unlikely if you are having emails sent to the same email server that you are using to send emails .. for example mail.yoursite.com is your outgoing smtp and email@example.com is the address confirmations are being sent to). 2) Your hosting company's email server is going down at times which would mean the confirmations are not sent at those times.
We have had both instances above occur although not frequently.