Sivir Spell shield

Sivir's Spell shield will sometimes proc to restore mana when you block an ability caste, but in two games the damage on two different abilities still applied. The only encounters I've had this were from Lucian's Q, and Varus' Q. For the record: I did actually prime the shield in time for both spells, but in these two instances it didn't seem to block the damage. I can't seem to duplicate the effect regularly.