There are a number of things that could have prevented a customer from making a payment. They are listed in order of most common occurrence as follows:
1.) Failure to have an updated browser. Customers who have very old browsers are unable to utilize the pay online service. Using a newer version of Microsoft Internet Explorer or equivalent will solve that problem.
2.) Having an updated browser but not having the “browser settings” for the SSL protocol. The SSL protocol setting is needed to access the pay on-line service.
3.) Encountering a corporate firewall. Many customers try to pay online during the day from their work location during lunch time for example. The entity that they work for may have a corporate firewall setting that prevents them from making any payments online.
4.) Pop-Up blocking software that you are running is preventing the opening
of the additional window necessary to link to the external website. You may be able to disable the Pop-Up blocker through a setting within your browser’s own options menu.
Online Payment Help Page