What the achievement asks
Sniff out five special items while exploring caves with a niffen companion. Five points, account-wide.
Two gates, and neither is explained in game
Special items only start appearing in sniffenseeking digs at Renown 9 with the Loamm Niffen. Below that you can run dig after dig and find nothing, and the game never tells you why.
The second gate is the weekly limit. Since the Midnight pre-patch you can only obtain three sniffenseeking maps a week, which puts five items at a minimum of two weekly resets even if every dig produces one. Plan for two or three weeks rather than an afternoon.
Sniff before you finish the dig
This is the mistake that costs a whole map. Each dig is a small puzzle, and completing the puzzle ends the dig. Players who focused on the puzzle first have finished it and been thrown out before ever looking for the special item.
Go in, sniff around, find the item, and only then finish the dig.
Once a special item is present, a quest marker appears on your map straight away, so there is no hunting involved and no chance of walking past one. What there is, is a chance the dig has no special item at all, which is why five items can take more than five digs.
Credit lands at the turn-in
The item you dig up starts a quest, and the quest is handed in to a niffen in Loamm. Progress only moves when you turn the quest in, not when you loot the item, so do not leave the quests sitting in your log.
Do the digs on one character. The weekly map limit is per character, and at least one player reports that maps run on an alt gave good reputation but did not move the count of five on the character they cared about. Reputation is welcome, but it is not what you are here for.
Where it fits
One of the pieces of Que Zara(lek), Zara(lek), part of A World Awoken. The Renown 9 it needs is on the road to Renown 20, which is There's No Place Like Loamm in another branch entirely, so the reputation is never wasted.