Azir's Ult sometimes not working

I was practicing Azir in 3v3 the other day and 3 times in 2 different games I ulted an enemy champion standing right in front of me (It was Pantheon the latter two times, I don't remember who it was the first time) and the enemy champion was not knocked away, nor did they take any damage. They didn't have any spell shields (I checked for that, no Morgana on their team, no Banshee's purchased.) They simply did not take damage or get knocked back by the ability. The part that made me really worry about it was that they then proceeded to walk through the wall created by my ult as though it wasn't there.