Arlathan Forest Shadowy Grove Puzzle Not Completing

Tried again after I finished every single side quest and companion quest in the game, and it worked. 
Glad there's a solution, even if it is a bit annoying. 

Thanks all :)

Good morning!

I was just experiencing the same issue. However, I checked another forum and they had the answer. :)

Do you have any conversation quests that you haven't done nearby? Like you, I completed the puzzle and heard the grinding noise that said the solution was correct, but the demon didn't spawn. Turns out I had a conversation quest for Bellara nearby that I hadn't done yet. Once I finished it, the demon finally spawned (I didn't have to redo the puzzle), and I was able to complete the quest and get the chest. 

Please post if this helped! (and a big "THANK YOU!!" to user "A" on the Steam forums for posting the answer!)

I just actually in this moment activated all the switches after completing every side quest and conversation that had or could be done in Arlathan and the demon spawn out the statue. so maybe that’s the thing with this, is you have to complete all side quest and convos in Arlathan forest. Not 100% about that but so far seems most likely. 

I hadn't done the conversation quest, so I tried doing the puzzle again after that. Unfortunately, it still didn't work. 

Maybe it's because in my save I had already read the letter prior to the conversation quest. To get back to a save before I read the letter would require me to redo hours of content. 

I'm happy this fix worked for you though and fingers crossed it helps others.