Why are my drawing objects under the Commenting Tool not visible when I draw them?

I am trying to markup a PDF in Adobe Acrobat Pro DC with revision clouds but when I draw one, despite it being set to 100% opacity with a red line colour and 2pt weight, I cannot see the cloud.  It says that commenting is allowed for the document under security and permissions.  I can't think of any other reason why this would happen.  I can select the shape and change its properties– I just can't see it.

Has anyone experienced this problem?