Markup “Erase” doesn’t do anything.

I know this question has been asked before, and the answer always refers to the technical language in the user manual, which is, essentially, useless. My problem is that on my iPad and my iPhone I simply cannot get the markup “erase” function to do anything. I tap “mark up“, select the eraser, and either the object or pixel setting, then proceed to try to erase something with either my finger or my stylus and nothing happens ever, repeatedly. I know how it’s supposed to work, but it doesn’t. Help!

My “workaround“ is to select the highlighter change the color to white and color over everything with white until the thing I want to erase is gone. Well this works, it’s lame, and I would like to have the full functionality of the app. Somebody please tell me what to do.