The most common reasons for this are:
Your Bank did not authorise the request. In this instance please contact your Bank.
You are using an expired/Cancelled Credit Card.
You may have reached your funds limit. Additionally you may have set a weekly spend limit previously. You can manage your weekly spend limits in the 'My Settings' tab once you are logged in. Please note: All player accounts have a maximum weekly spending limit: $10,000.
Useful link: My Settings