Unable to set consumable items.

Sometimes after depleting potions, either HP or AP potions, when I put items in the assigned shortcut numbers, the potions doesn't set.
In order to set items again in the shortcut number, I have to remove almost all skills from shortcut numbers( QUICK option ), then only I'm able to set potions again.
It doesn't always happen.
But sometimes it happens one after another and it gets really annoying, specially when it happens in the middle of fighting mobs.
Is it some kind of bug?
Is there some way to fix it?

What I normally do is put the pot in the 2nd quick slot (alt+q), then left click and hold it and back to quick slot 1(alt+q) then drop to the desire shortcut