"Practice weak skills" doesn't increase any skills
When choosing to practice weak skills sometimes all I get is XP instead of raising the bar on any skill level, which is kinda disappointing.
The bars on the skill icons represent an average of the word strengths for that particular skill. When you practice a skill, it will work out a new average and if that takes you over the minimum value for making the skill go gold then it will go gold, but it is just as likely that the new average will not get above that limit. It will all depend how low the average was and also what words were strengthened during the exercise.