There's only 3 things I can think of that may be the cause.
Overwatering will leach-out nutrients and promote root rot. (does your rootball stink like rotting food?)
Losing buffer strength in the soil is a strong possibility, too. Tiz why I do my final transplant about two weeks into 12/12. Adds fresh ph buffers (in the fresh soil) to help with keeping ph in line, and avoids rootbound issues later.
Calcium might be the ticket though. 1/2 to 1 tsp unsulfered molasses per gallon of properly ph'd water, once a week. Provides calcium, magnesium, iron, and a few other handy micronutes. Plus, it is a carbo-load base ingredient. (carbo-load products, CalMag Plus, and many, many other "MegaBud" products are a watered-down molasses or synthetic molasses)