It could be one of two things.
It might be that your payment hasn’t passed all our security checks. Please ensure your billing information all matches with the card/PayPal account you used and go for it again.
It might be that we’ve run out of the item you ordered (it happens sometimes ☹) .
If either of these have happened, we’ll contact you and let you know what’s gone on. If not then get in touch firstname.lastname@example.org with your email address and order number and we’ll investigate.