Did you have enough ferts and all that? My 15 gallon had almost no growth, then I got the Seachem series, Seachem Flourish, Trace (Plants not the fish one), Nitrogen, Potassium, Phosphorous, and I plan on buying Iron for my other plants because now I can see the difference in growth and reduction of algae. The higher light burns through so much more ferts because the plants are using more of them.
So I does Macros (Nitrogen, Phosphorus, and Potassium) and Micros (Flourish), then trace for all the other depleted minerals. I am following someones on Youtubes routine. Works quite well. PM me if you want to go more in depth or what ever.