Your light may seem bright to you, but it is not enough for the HC. This thing likes TONS of light. Add light any way you can.
If you can suck out all the water and leave the HC in the wet Aquasoil (no water) you will have better luck. If you want to have even better luck then once a day, every day, pour water in the tank to cover the HC and then immediatly drain it completely so the HC is again in wet AquaSoil only. Fertilize this water liberally.
Basically the best results come when the HC grows in moist substrate. Lots of Oxygen and so on. HC hates being submersed, haha!
Here are some pictures of my emersed HC. Submersed HC never gets this big and healthy: