Let them dry a little more between watering. Without oxygen the roots can't take in water, showing symptoms similar to nutrient burn. You should let the top inch of soil get bone dry. There's still plenty of moisture still down in the root zone. I hate those meters, the one I had was worthless. You don't really need them. when they start to dry out the cups will feel noticeably lighter. Right when you water them the roots are suffocating, when you let them dry out oxygen returns to the root zone and they begin drinking again.