Ok, first off that is not a normal setup for such a small snake. A 20L shouldn't be used until they're at Least 3-4 Months old.
A snake that little should be in a 6qt or at most a 12qt tub with a little water dish and coconut half or other small hides. That snake is probably Majorly stressed out.

On the hides issue, I have noticed that snakes coming from places with no hides or improper hides usually avoid any hides in their new home until a few days have passed and they 'realize' the hides are safe, dark spots.

Go to WalMart and pick up a 6 or 12qt tub ($1-3), melt some ventilation holes in the upper sides and lid, get some binder clips or mini bungee cords to secure the lid, use some flexwatt or a 1-5gal sized UTH to heat one side underneath, put in some paper towel, a little dish and hides, and leave them totally alone for a while. Things should improve for the little one in a smaller enclosure.