You should check in your options at the top right of your Board, by clicking on the, that in the "Participant settings" box, "Free" is selected. You can also choose if they can act on the drawings, images, shapes, and/or ideas of others..

If this setting is made on your side, your participants have to go to the "Interaction Options", the arrow on the bottom left of the Board, to choose "my ideas and ..." to be able to place their ideas on the Board themselves and act on them.

If you have allowed it in the previous setting, they will also be able to choose to act on the drawings, images, shapes, and/or ideas of others.