You may be being fooled by the lack of automatic refresh. Sometimes lines that have been erased may still appear to be there until you make the screen refresh. I usually just zoom or move around, but you can click the refresh button (the double blue arrows at the top right of the toolbar near the zoom tools). See if that works.

The trouble is that I can't see what you are talking about to be much help if it isn't just the lack of automatic refresh causing the problem. Are you able to upload a screen shot?

As a general tool tip - When using any of the tools, if things don't seem to be happening according to your expectations of what should happen next, its always best to check what the message in the command line is saying at that precise moment in time