Leona's Q makes her next basic attack deal additional magic damage for a certain amount. This works on turrets aswell. It doesn't work on inhibs and I'm pretty sure it's intended (eventhough Wiki says it works on structures and inhibs are structures aswell). However, previously if you would hit Q and attack an inhib, it didn't 'consume' the Q. You would just normally AA the inhibitor and the Q mark would stay on for 6-7 second or untill you would hit a target that could be affected by it. Since last patch the Q will be consumed when you attack the inhibitor but it won't apply the damage. Therefore you can't press Q and save it for a target while attacking the inhibitor.
**TL;DR:** Leona's Q get's 'consumed' when attacking inhibitor.