When you edit items, you may have different forms open. You can only edit one item at a time. The item you can edit is shown in the top most, active form. You can recognize an active form by the color of its title bar. For example, an active form has a blue title bar and an inactive form has a grey title bar. The title bar colors of active or inactive dataforms depend on the display properties of your computer.
To make a form active and ready for editing, click on the form's button in the task bar.