Reinekii Deficiency

Maybe it will look a bit better if you increase your nitrates to 25 or 30 ppm, that seemed to help for the ARs in my tank (as well as all the other plants).
But looking closer at the pictures, the most likely culprit is right in the middle of it. Occasionally Amano schrimp actually develop a taste for ARs, especially if there is not too much algae or other food in your tank available. Feeding them some blanched lettuce or zuccini might help to stop them frenzying on your plants.
I had a healthy AR stem shredded overnight once by Amano's, right at the point I started nailing the lights, ferts and CO2 which got rid of most of the algea and the tank never looked any better!

 

You might also want to try urea as a nitrate source. I have had AR and AR Mini (which I've grown well before) completely dormant for months in my tank with black sand substrate. After three days of urea dosing they look a lot better - we'll see if it continues. I agree it looks like you have a lot of snail damage. Even when the plants have not been growing, I had been dosing 10-20 ppm a week of KNO3, plus some Osmocote in the substrate. Calibrated test kits showed 25-30 ppm nitrate, but there still could have been some error there. Would even more KNO3 helped? Maybe. But so far the results of using urea are promising.