Connectors keep reverting to arrow head at both ends on re-opening an Impress file.

When I re-open a saved Impress file the connectors keep reverting to arrow head at both ends, even though I have edited them before saving. Also, if I select to use a connector with one arrow head, the result is always a connector with arrow head at both ends. Is there a way to stop this behaviour or am I doing something silly?
I’m using Version: 5.0.4.2 with Windows 10. (UK English). [I’m new to LO and so far I think Impress is much better than PowerPoint].