Tell Me Why

Missable achievement if you skip doing the barn loft's wall puzzles and go straight to opening the locked box.

When you first enter the barn loft, you will find a giant puzzle against the back wall. To solve it, press these sections of the wall (all sections are described from your perspective):

-The creature's left hand
-The small rounded tree above the creature's right hand
-The princess in the tree to the right
-The castle on the left
-The sign at the very top

This will open a compartment in the wall. After examining it (and getting the 13th and final collectible), you will unlock a compartment to examine as well as the ability to do the next wall puzzle.

Turn around 180 degrees and look at the center wall puzzle on your left. It is blue in color. You have to slide tiles so they hover over certain pictures to complete it. From top the bottom:

-Move the top slider so the hole is over the dove
-Move the next slider to the far left so the hole is over the ballerina
-Move the next slider to the far right so the hole is over the lady in the dress
-Move the bottom slider so the hole is over the man in the overalls

This will unlock another wall compartment to examine and the next wall puzzle. Do another 180 and face the next central wall puzzle. This one is a series of objects you need to click in a certain order. Click these:

-The castle in the lower left corner
-The flowers at the top center
-The clock in the bottom right
-The house and tree on the far right side
-The dinner party in the center of the image

This will unlock the next compartment of which there is a paper inside. Read this to unlock the achievement.

P.S.: to unlock the lock box, the combination is leo (all lower case cursive letters).

In the chapter 'Princess and Tiara', you will be in the barn loft with a bunch of puzzles to solve in order to open a box. Make sure you solve the puzzles instead of breaking open the box.

The first puzzle is located on the back wall and requires you to press the following buttons: Hand, Tree, Princess, Castle, Title.

The second puzzle is on the left side wall and requires you to slide the pieces to these positions (from top to bottom): 1 left (Dove), 2 left (Ballerina), 2 right (Engineering), 1 right (Painter).

The final puzzle is just behind you and requires you to push the buttons in this order: Castle, Flowers, Clock, House, Dinner Party.

You will unlock a secret compartment with a story in it, earning your achievement. You can replay any section of the Episode in Isolation mode without overwriting your story progress.

During the mission princess and tiara, you will need to solve 3 puzzles while in the attic of the barn to get this achievement!

Here is a video guide with all the chapter 3 related achievements and collectibles! The puzzle solving for The Princess' Lost Treasure Achievement begins at 3:36! Timestamps for the other achievements and collectibles are in the description! Hope this helps!