Cannot select arrow that was previously "Sent to Back"

Usual suspects:

• unselectable item is not in the apparent frame, but on the page - this can be tricky to fix
• graphics frame inside another frame: item isn't in the frame it appears to be in - send apparent frame to back
• some other element has Run-Around turned on - turn it off

Is the flowchart inside an anchored or graphic frame? If so, then select the frame and use ctrl-A to select all, then try to click on the arrow to make it the only selection.

If you haven't usd frames, then just do a ctrl+A on the page to select everything and try to select that arrow. WIth a lot of objects in front of it, you may have to de-select the others first (ctrl+click).