Hi, I'm not an expert like others, but I had the same problem in my first grow and in my second too, and I figured out the problem just yesterday. My problem was that, because I was scared about overwatering, I was underwatering my plants, so they were always hungry for water and nutes, your lower leaves can be a symptom of chronic underwatering too. If your soil is always dry when you water them and you are not giving too much water, well can be the same problem!
Hope this help.