Rengar Empowered E- Bola Strike - Bug

This happened to me a few games ago while I was playing Rengar. When I used my E at 4 Stacks to get my 5th Stack, It would stay on cooldown instead of being available as an empowered ability. I don't know if this is happening to other people and it only seemed to happen when my E was level 1. If anyone has any ideas please help.