Slitherine

So I set my reaction arc for a unit with ranged weapons - then watch as enemy units march through the arc and my unit does nothing
what am I missing ?

Some weapons don't have reaction fire. Flamers, las cannons and grenade launchers don't.

When the cursor is over the reaction fore icon it shows the tiles that are covered if you face that direction. You can't reaction fire through friendly units even if they're adjacent unless you're on higher ground.