I grow a ton of this stuff, the comment made about change is likely the key there.
I cooked a tank and poushed it to 90F, that melted most of the leaves over the next 2-4 weeks, then 1-2 weeks after that, new leaves came back.
It's a bit like a Crypt really.
This has nothing to do with ferts, Ca++, CO2 only if things are bad for a week or more/chronic etc.
Try trimming the new tops and the bottoms will sprout new growth.
When you plant this plant, it does nothing for 1-2 weeks.
It's growing roots actually.
So it does not bother saving those leaves you have when you get the plants.
It'll just grow news specific to the tank.
My tap is KH:1 and GH or 2.
I dose about 1 teaspoon per 60 Gal of gH booster 1-2x a week.
It's nothing to do with Calcium, I've never seen any confirmed Ca++ deficient aquatic plants in my life. Well, Rice, but no submersed species.