My stone retaining wall has slumped over time:

https://www.dropbox.com/s/2khk9wws7imeg0t/2020-06-09%2018.28.03.jpg

https://www.dropbox.com/s/8oi0dzuvxsutdt5/2020-06-09%2018.27.42.jpg

It may be a little hard to detect in pics, but it has sunk about two to three inches in middle :--(

I don't want to tear down and rebuild, nor can I raise area from below, e.g. via mud-jacking. Is there a standard or an accepted way to build up/fill in the slumped area so the slope from left to right is again uniform and straight? Aesthetics are only marginally important...