What? It doesn't do that to me!

Seems to do that if I get a lot wrong or used many hints in the strengthen. Probably working as intended.

In order to move from 4/5 to 5/5, you need to increase the average strength of the words in the skill.

Usually, one practice will be enough to raise the skill to 5/5, but not always. Sometimes your practice doesn't raise the average percentage enough to meet the cut-off point and you have to do the skill over.