Passive meditation only occurs in medable armor. Non medable armor allows you to only have natural mana regen a which is what everyone has regardless of the meditation skill.
If you have meditation as a skill, you need to wear medable armor to actually use it actively or passively.
lil correction to this, you can actively meditate in studded leather. It just takes ages.cloth and regular leather give you the full meditation bonus. studded leather, bone armor, ringmail, chainmail, and platemail give you 50% natural meditation and you CANNOT active meditate.