Float it in the water of a well-lit, Co2 injected tank. Drop it in and forget about it for a few weeks. Then take a look at it. It seems to grow faster floating and it starts to grow roots down into the water. Then it's easier to plant.
HC requires PATIENCE based on my limited experience with the plant.
"This ain't your momma's glosso."
"Insanity: doing [or asking] the same thing over and over again and expecting different results."