Varus W is currently bugged.
Currently Varus can't detonate his blightstacks if his target has a shield.
It has a 100% chance to reproduce, just go into a custom game, pick Janna and varus on the enemy team.
Have Varus stack his blight on Janna and have Janna user her shield against Varus' E or Q. His ability will damage the shield but the blight will not detonate.
It happens with any kind of shield, i tested it with barrier too.
Also his critical strike animation stops working for the remainder of the game if he skills up his W.
Please Riot look into this, i doubt any of this is intentional.