Sometimes, when I try to strengthen a skill, even after a successful practice the skill is not strengthened. Do you need to succeed with full lives or something? Thanks
If you are doing a non-timed exercise, you have to complete it, but you don't have to complete it with full hearts. If you fail a non-timed exercise, you do not get any strengthening credit.
The easiest way to strengthen skill is to do a time practice. Even if you don't get though the entire exercise, you do get credit for work completed.
If all the words in a Skill are weak, it may take a couple of 'strengthen skill' to improve all the vocabulary in the Skill. On average, each 'strengthen skill' will improve only 10 vocab words.
Sorry but I don't see the option for timed practices, and I also have never seen a timer or anything
You have to buy them in the 'Lingot' store. If you are in the new format (blue toolbar), I think you access it by clicking the lingot (red gem looking thing) on your task bar. If you are in the old format (black toolbar), go to your home page, and right above your tree, you will have a tab for 'store'.
I think it cost 10 lingots, but it is well worth it.