The Fold window in Studio Toolkit for Boxes will not hold the designated creases.
When folding a structure, the Fold window shows all panels as expected at the desired creases. But after saving and showing in Studio Designer, some creases are lost and the structure is not folded correctly.
First, use the Check button in the Fold window to check for double lines and gaps.
If this does not work and the folds are still lost when seeing in the Studio Designer, check your paths (especially crease path corners) to ensure that the paths (and not the strokes) match up point to point. Use the Adobe® Illustrator's® Outline Mode to view the actual paths as viewing stroked paths is never accurate.
The 'Check' for gaps has a specific tolerance to be set, so there may be corners that will still cause problems if not addressed.
You can optionally also use Illustrator's "Object > Path > Clean Up..." menu to clean up stray points as these will also be taken into account when assigned a crease or a cut stroke.
Make sure that a crease really touches a cut line and that crease and cut lines are never split up into separate (even though connected) smaller lines.