If the skull is full with the mouth open and tapping it does not work, the issue might be related to line of sight. So you might need to re-position yourself if you are on a bridge or your enemy is around the corner.

The skull might not open even when it looks full for a few different reasons. Your enemy might be in the process of spawning in or being knocked down, or you might need one more kill for full feeding power.

If you are experiencing something outside of the listed conditions, please post the following:
1. What mission are you on?
2. Are you playing multi-player or solo?
3. Is the skull failing to open when full, or is your character failing to feed while the skull is already open?
4. Are you fighting a boss?