If you can not see the symbols you have assigned to a drawing, the reason can be that the layer you assigned the symbols to is deactivated.
Detailed guide
Click on the button “Define and select pattern” in the tool bar. (1)
Click on the button “Turn layers on and off for visibility” (3) on the tab “Symbols” on the right side of the screen. (2)
The window “Visible layers” now opens. If the layers with your symbols are not visible, mark it by checking the box left to the layer. (4)
Click on the green tick to finish the process and close the windows.
For more information about symbols,
watch the video about color coding and symbols in the online tutorials → here