The lower than optimal humidity is most likely the cause. Try and get it out 50-60% normally, and 60-70% during shed.
There are a couple ways you can raise the humidity and help out future sheds.

This works wonders and is super easy.
Try a different substrate that holds humidity better (newspaper, repti bark, cypress mulch).
Add some damp (not wet) sphagnum moss to the enclosure.
Mist the enclosure every so often, but this can lead to bacterial growth and mold.
Run a humidifier in the room.
Or, make a humid hide. Cut a hole big enough for the snake to fit in a plastic container. Pack it with damp sphagnum moss. Place in tank near shedding time.