I have a 'center-line' I need to move a few inches, and one dimensions attached to it. But when I try to modify and/or delete them, it just does not happen. I can move the dimension around a little, or reattach it to other geometry, but I can't delete it. I can only hide it and make another if I want to change anything.

How does this happen, and how can I undo it when it does happen.