are you phing the water going in? soil should be 6.6-6.8, soil-less (promix, sunshine) should be 5.6-5.9.
that is the first thing I would check. then I would check to make sure you are not over watering. they should be bone dry and very light before watering again. next I would check that there are no root aphids as a result from over watering. good luck!