Sounds like you got ahold of a bunk batch of promix. I been using it for years and I never had a problem like yours. It doesnt really matter what your ph is in the water your feeding the plants. It always comes out with a runoff in high 5's usually 5.8 everytime. I done several experiment tests even put in just straight tap water from faucet and it still gets a run off high 5's. On the bag of promix it only guarantees a 9 month shelf life once it is open and also if you are mixing your promix with water before you put into pot you have to throw out all unused left in the mix. The water activates the lime and cause the ph to rise. So only premix what you plan on using and never store the wet mix