When all of the placeholders on the page already have components, you will not see the large, empty + Select Component placeholder. You can still add components to the existing placeholders with the following steps:
At the top of the page, click on the Home tab .
Click the button labeled Component.
Multiple buttons labeled Add here will appear everywhere on the page where it is possible for you to add a new component. Scroll to the place you want to add the new component and click the Add here button.
A window titled Select a Rendering will appear with a grid of components for you to select from. Click on the component you want to add and then click the blue button in the lower right of the window titled Select.
Your web browser will automatically refresh the page and the new component rendering will be in the placeholder you selected.
Click the green + Add Component button.
The component you selected has now been added and automatically saved to the page.