Difficult to see from the picture what is up.
Assuming the other bolster is fine, I would get into the cabin, facing the headrest, and feel around the back of each bolster to compare them. You should be able to tell from that whether the problematic bolster needs to have the edge of the leather pushed in further, or whether it has popped out and slipped over the outer edge of the seat back. If it's the latter, then you probably need to pull it out a little more, re-align the plastic bead on the leather with the slot that runs around the back of the seat, then push it (hard!) back into place.