Lulu's E

Sometimes, like every 40-50ish game I noticed that her E does not work. This is true both when using it on allied or enemy targets. It goes on cooldown, there's a sound effect too, but it doesn't do anything. I'm sure it's not just me imagining things, since I randomly put a shield on my ADC while leaving fountain (no targets to attack her) and it didn't appear. I don't know how to reproduce since it doesn't seem to happen in any specific scenario.