Lux Ult

There seems to be something weird going on with Lux's Ult. It's not rare but slighty. it might happen twice a day or so. Whenever I (or another being) Ults we aim right at the champion and that champ cannot move because of snare.. but the ult is going through them but it is not damaging them. It's like I didn't even ult. no damage whatsoever. Can someone help me on this.