That's a tough one - I'd be interested myself. On reading some time back, there was a suggestion that the plants don't like the water - the base dies off and they float somewhere else. From Tom Barr I think but a while back and was just glancing over posts. It might have been the plants adjusting. Is the new growth in your tank water fine? Also are you dosing any glut? Generally I find lower leaves drop off, stems are weedy and no sideways growth if low light / low ferts / low CO2 but stem will still persist. Has to really hate something in water to rot out (assuming not squashed on planting but ambulia should do well generally). Anyways not much help I'm afraid.