Some of the skills have a lower skill that you need to unlock first. For example, in the case of the Level 4 boost, you have to first unlock the level 1, 2 and 3 boost. For Mad Monkey, you have to first unlock the "Take that!"
Some of the skills have a lower skill that you need to unlock first. For example, in the case of the Level 4 boost, you have to first unlock the level 1, 2 and 3 boost. For Mad Monkey, you have to first unlock the "Take that!"