Hi mate, this is a PH issue. I would not use rain water for the simple fact it may contain pest eggs/larvae and impurities which will affect the PH. Try and keep her around 6-6.5 and maybe do a slurry test of your soil to check the ph your roots are sitting in. Water temperature is also something you need to watch. Between 67-72'F is optimum. Any lower and she will struggle to absorb nutrients. Any higher and the roots struggle to absorb oxygen. To lower the PH of your soil you can use a small amount of agricultural gypsum, dolomite lime to raise it.
I hope this helps, good luck and happy growing.