Hygrophilla Deformis should root like a weed.. is it rotting off at the stem? also are you dosing ferts and providing any carbon?
It should root in any substrate, fertile or not.. its equally able to feed from the water column or the substrate so a fertile substrate isnt necessary. Be careful when you plant it not to damage the stem, its very prone to rotting off at the bottom if it gets bruised.
It could also be fish pulling it up.. I seem to remember my barbs loving to do that!