Water parameters of the tank it's in? Lighting specs? Could you give us more info on the tank setup in general?
I keep spare clippings of Hygro in an unheated tank with no light other than what comes in through a window and the stuff still grows for me lol.Hygrophila polysperma requires a good lighting from my personal experience. I tried it in low light and I lost it after a couple of months.
Michel.