A customer bought my downloadable product but says the download doesn't work. Why?

The message the customer sees explains the reason:

• "This download will be available once your payment is confirmed." The order is still unpaid or waiting for payment verification. Confirm the payment and update the order status in Orders → Manage Orders.
• "This order was cancelled or refunded — the download is no longer available." Downloads stop for cancelled or refunded orders.
• "Please sign in to your account to download this file." The customer needs to log in to the account they ordered with.
• "Download limit reached for this file." or "This download link has expired." The product's Download limit or Expiry (days) has run out.