Sivir's Spellshield

I noticed while recently playing a games as Sivir that her spellshield wasn't correctly blocking abilities. An enemy Sona's q would land and even though the spellshield was up, and correctly timed, the damage would still apply to me. I knew that the shield was correctly timed due to the fact I was still getting the mana reward. This occured several times, but inconsistently. When paired with other abilities to be blocked at the same time, the results varied. I don't know if this was a isolated incident, but I figured it'd be better to make sure it isn't recurring.