That dashboard display is for the traction battery, not the 12V battery. To get it to 100%, you may need to use regeneration down a long steep hill. Don't be concerned about never getting there on flat ground, the car is trying to manage the battery for long life. Don't defeat it.
Only if you are going down a long steep hill and use regeneration, then it will go clear to the top (8 bars). During normal operation it is designed to stay between 2 and 6 bars, usually 5 or 6.