I guess it might make it easier for GOG if we compile a list of achievements that have failed to trigger. For me, I've had problems with the following:

* Marathon -- My character has run almost 200km and still not received the achievement.
* Dare to Devour - My character has eaten everything available, including all new recipes, and still nothing.
* Treasure Hunter - I'm at the end game, with all chests I've seen opened once (bar a few that need the teleportation spell) and still no trigger.

(Secret achievements)
* Wing Clipper - End game... still hasn't triggered.
* Speed Demon - Same as above

For me, the secret achievement Dragon Slayer, and the regular one True Haggler haven't shown up. I'm at the end of the game and I've sold 3 mil worth of stuff and I'm at the end of the game. I'm missing a few things like a scarf, so I can't say anything about Item Collector yet.

I've played the piano with the fairy familliar in my original game, NG+ Hard, a fresh normal, a fresh hard, and a fresh nightmare. I've wiped and reinstalled the game. I still can't get the Recital Achievement to trigger. :(

Started a new game plus. Somehow Wing Clipper decided it wanted to pop after I defeated the correct boss.

Also, add one that didn't pop: Usurper. Even though I killed Gebel several times. Missed that.

It seems to be kinda random because i've gotten most of these achivements. I'm missing the piano one, too, though.
EDIT: got the piano one, i just had to wait until miriam stood up by herself

I'm having trouble with the hair style achievement.

Also have issues with Dare to Devour. Just tried devouring 20 things in one sitting in a New Game Plus, but it didn't unlock there either. So I guess a new game that's not New Game Plus is needed at the moment. :/

Also Snake Charmer didn't unlock for me initially, and a Boss Rush of it didn't solve it. New Game Plus worked there though.

Regarding Usurper maybe you have to have the Zangetsuto?

Did it both ways.

Achievements may not trigger if having started the game directly via the .exe instead from inside GOG Galaxy. I missed 3 early achivements because Galaxy was not running at the time those achievements would normally trigger. I then started a new savegame (while Galaxy was running), et voilà: I got those 3 missing achievements after having advanced to the places where they occur.

I'm missing True Haggler, Growing Pains, and Bookworm, meeting the requirements for each of them.

Contact GOG Support and open a ticket about this issue.