Write on behaviour issue

I have just tried to do a paint stroke that is revealing text . I am using an image mask to do this. I created a zigzag line with the line shape tool and then applied a write on behaviour to the shape. Following this I created an image mask on the shape (pain stroke) and put the text in the source well. My problem is for some reason my pain stroke will write on but will not erase afterwards. This is in spite of the fact I have selected it as the designated option in the HUD. Is it because I am using an image mask that is causing it to only write on but not erase? I have since tried to use the paint tool and also a shape with a write on effect and both are not erasing as they should. Any ideas?