Sometimes scorpions do not molt properly. This can happen because a scorpion is not fit enough (the process of succesful molting requires the animal to be fit and healthy) or if the captive conditions are not properly set. The humidity and temperature should be in the ideal range for the animals to molt.
The molting process is a critical stage in the scorpions life. Problems can occur before, during or even after a succesful molt.
Here you see a picture of a young H.trilineatus that did not make it from instar1 to instar 2. If you look carefully, you can see that the carapace opened up, but the process stopped there.
Picture is copyrighted by R.Groeneweg c 2007.