Hecarim E Bug

So I was playing a ranked game and I was laning against fiora. About 3 minutes after I used my E when she was low hp (about 150) and it did the animation but there was no knockback or any kind of damage dealt. Today this already happend to me twice** (and no she didn't use her W to block my damage or my E didn't run out of time).** Fiora also noticed that and she also told me it was a weird bug. note: this doesn't happend everytime but the small probability still there