We do not know why your payment has not been authorised. This is something that you may need to contact your bank about.
If you are attempting to make payment for a substantial amount (e.g. paying in full for Tuition and Accommodation Fees ) this may be over your daily limit. Try paying for them separately on different days , or contact your card issuer to amend the limit.
If you are on campus you can make payment in person at the Student Services Centre. Card issuers apply different rules for payments made via the web to those made in person
Details of all available payment methods can be found here