Dean,
It would be ideal to take readings from 30 minutes before gas on until gas off at 30 minute intervals. Then you will know what the unsaturated value is prior to gas and getting a datum point at lights on will tell you how effective your dissolution is. Again, "Ideally", assuming your KH is medium to low, there should be a 1 unit drop. from before gas on to lights on. We can only rarely ever achieve that without causing the CO2 levels to reach toxic levels further into the photoperiod. Your problem may "simply" be that you're not saturating the tank with gas early enough into the photperiod. This is actually the most common problem. If that isn't the cause then we have to look deeper at the setup.
Adding liquid carbon is a very simple verification of CO2 deficiency. If the plant health improves immediately as we start adding the liquid then we know that CO2 uptake is a problem. If it helps then you just keep using it until the gas is sorted and then it can be gradually withdrawn. Dose 2X or 3X the bottle suggestions because you have no fish and I think those plants have a high tolerance for liquid carbon. Carpet plants and most stems, even mosses, respond well to liquid carbon.
Your plants looked OK in the earlier stages so the present configuration worked at a lower plant mass, but it seems to have faltered as the plants got beefier. There is more blocked flow and the demand is higher due to higher mass. Filters may have gotten dirtier and more blocked over time. Try removing some media, especially if it's chocked full of media. Keep it unblocked with frequent cleaning.
Cheers,