Trigger Pane - Down arrow has disappeared

Hi,

This might be a daft question... but my down arrow has disappeared on the trigger pane, so I can only move triggers up and not down to re-order them.  Is there an option to enable/disable this button? - if there is, I can't find it to turn it back on!

I'm using Storyline 360 and I've just looked at colleague's and they definitely still have both arrows.  I've tried different story files and also having different triggers highlighted, so there is room for them to physically go down - but it doesn't re-appear.

Help??!! Please :)