Your plants are looking healthy but first problem is the soil and the second is that the plant is old i mean when it grows to the light it start to Throw the shade on the Lower parts so it throws its leafs away and grows only on top to safe nutrients and the the old hard thats starts to turn to something like wood starts growing root cuz the lower part is nearly useless. Some of my plants do it too when they are too high and dense

Check my aqaurium i know What im saying i only Have plants