There could be a few different reasons for this to happen:

• Mixing up Module – Course – Examination
  Please note that you always have to sign up for a module first and then for a course. When you signed up for both module and course, you are not automatically signed up for the examination. You have to register for the examination within the registration period separately.
• Missing Confirmation
  After having chosen the examinations in the list and having clicked Register, a new page with examination details appears. Here you have to click Submit to confirm the registration.

For more details, please read our tutorial explaining How to Register for an Examination.