erase 3D items in the exercise file

[franklawton64]

Why does the subject command fail to erase the filled-in 3D items in the exercise file titled Design_Line_Anatomy.dwg? (copy attached)

[franklawton64]

I used laydel to purge the stubborn 3D items and the erase button worked again after I refilled the design line. I am not sure if this is the right way to do it, however.