the strengthen all skills button hasn't been working for me for a couple weeks
Occasionally I'll get the Skill Strengthened notice after the lesson, but it's rare. The strengthen buttons in the individual lessons are working fine.
When you use the "Strengthen Skills" button on the home page, you won't always strengthen enough words in a single skill to strengthen a skill.
The way the global strengthen skills button works, is it pull about 8 words from your total list of weak words (these may be 8 words from 8 different skills), and strengthens those words. If the skills that those words were pulled from only had one weak word, the entire skill gets strengthened. But, if that skill had many weak words, and only one word was strengthened in the practice, the skill won't turn gold.
The more skills you complete, you will find that it often takes several global practices a day to strengthen a skill.