i had this same problem except my pH was more around 7.4. i flushed one plant with about 1.5 times the amount of water (around pH 6.0) as there was soil in the pot. with the other plant i just made sure to water with an acidic solution. the one i flushed yielded less. in your case since the ph is so high i dont know if just watering with acidic solution would work. hopefully someone with more experiance can help. also i think you can add garden sulfur to your mix to lower ph.