I like your shading of the folds, they are very subtle and soft.
A little tip to prevent illustration blindness: Select all layers, turn them horizontally. Switch a few times between them, you will almost immediately spot any flaw.
In photoshop I use these shortcuts: [Ctrl-A] [Ctrl-T] [Right-Click] then select [Turn horizontally]
then use [Ctrl-Z] to switch between the turned versions.