What am I missing to find arrow heads? apparently the metal detector doesn't work.

[Turmoil]

no matter how long I search, canvasing the area shows nothing.

[969rob]

may be your map is out of date

[Mr.Softy]

Arrowhead spots have this golden glitter in eagle eye.

You don't need a metal detector for arrowheads, just the shovel.

[samcharles77]

Sometimes there is a bug where the collector item doesn't spawn even though the map tells you to go to that area. Try jumping to a new free roam session the item should spawn then.

Arrowheads are a mix of ones that can be dug up with shovel and others that need the metal detector. A single map may have you searching for both types.

Also there is an ability in collector that makes arrowhead mounds glow in eagle eye. At a certain level it unlocks. Before that they will not glow in eagle eye at all.
My advice would be not to do any arrowhead maps until you have the shovel, metal detector and the ability. Otherwise finding arrowheads can be pretty frustrating.