I agree with both Boredomb (his last post) and Mikaila. There is (in my view) clearly insufficient nutrients to balance the light. What does surprise me though is your comment that algae is a non-issue. In the circumstances described, I would expect algae issues. It may be that the fast growth of Wisteria (which you say is doing fine) is helping to stop algae, but few if any nutrients are then left for the relatively-slower substrate plants.
And CO2 is probably the nutrient in least supply. Tom Barr says CO2 in a low-tech system is generally "gone" by mid-day [in terms of the tank light period], and of course this varies depending upon the fish and organics. Most CO2 in a natural system comes from the breakdown of organics. Which brings me to the substrate: don't vacuum it during water changes, leave the detritus to work down into the substrate where it can be broken down. Malaysian livebearing snails help a lot with this, as they break down the larger bits fast, making them more easily obtainable to bacteria. And all this is your major source of natural CO2. The fish load also contributes, both in direct CO2 via respiration but also in increasing the waste in the substrate.
Other nutrients may be missing too. But I would not use Flourish more than twice a week, it is not intended as a daily supplement. You could consider other nutrients such as dry fertilizers and substrate fertilizers. The latter would probably be more effective in this situation. Adding more nutrients to the water column with CO2 minimal is not going to help, and may well cause algae.
Reducing the light period might also work. However, a caution here; Wisteria is a high light plant, and doing this might well cause it to start falling apart (lower leaves). My swords, crypts, vallisneria thrive; Wisteria rots to mush due to inadequate light. Aquarists sometimes forget that not all plants will be compatible for many reasons, including different requirements respecting light.