This squire common. It is caused by the depletion of nutrients in the tank. Nutrients come from fish tap-water, substrate, and fertilizers. Your tank initially had nutrients and the plants did well. However over time the nutrients ragout and then the plants stopped growing as a result. I have seen this a number of time and in each case I was able to identify the cause and fix it. But in some cases it took me months to figure it out.
What are other differences in setup, substrate, number of fish, , and water changes (volume and frequency)? In all likelihood the small tank has more nutrients in it then the larger tank. Flourish uses iron gluconate that only last 1 to 3 days before it drags and the iron rusts. Once that happens the iron is not soluble and unavailable to plants. It would be better to break up the one dose into 14 smaller ones. But that said iron may not be the problem. You may also want to try increasing the dose amount. There are 14 nutrients but if only one is short the plants will suffer, stop growing, and may die.
The maximum power rating of the tube light is 8 watts. Not enough to do any harm to plants. The other lamp doesn't have a power rating so I cannot say. However in my experience with a light that can 300 lumen to 3000 at full brightness, high light has never harmed by floating or submerged plants.