Your water has way too high of a hardness. You may want to look for
anything that may be releasing Calcium in your water - stones,
decorations, zeolite chemical media.

Hardness may be the problem. But I've seen Riccia refuse to grow in a
CO2 injected tank, with 3.5 wpg of 10,000K VHOs on top, and soft
water. What they did was to change the Riccia with a completely new
batch, from a different place. And it still wouldn't grow. The only
other plant in the tank was dwarf sag that grew very well. The water
in that tank had low nutrient levels, but that is valid for all the
tanks that that company maintains and some of them have very lush
Riccia carpets.

You may try reducing the KH first.
Also - making sure your CO2 is at least 20 and that you got the N, P,
and Fe right. I personally don't think Traces and K affect the Riccia
as much as the N, P, and Fe.
If none of the above works try a new batch of Riccia, totally
different than the one you have.

--Nikolay