I have 4 Lowryders at 4 weeks old. 3 are fine and healthy looking but 1 has turned yellow and droopy and isn't growing anymore. All 4 have same water, same pots, same soil and obviously the same light - 400w hps. They were all from the same supplier in the same batch. I can't understand why 3 should thrive and 1 start to die - any ideas?