Problems: Bring to Front / Back
 

Help please! I'm editing a document in Omnigraffle 5.0.2 and one shape has disappeared to the back and I cannot get the shape to 'bring to front' or move the shape above it to 'Send to back' . The option is there on the menu and does something when I click it once but still doesn't bring the shape I want to the front. Any clues? Am I being stupid?
Thanks in anticipation.