sounds like a Potassium def. spotted lower leaves usually signal this.
Either your PH if off, stopping the absorption of K, or you need to feed it more potash rich fertilizer.
Im not sure, but some micro-nutrients can also cause defiances in absorption and utilization of macro-nutrients... and if your PH is off, it will effect the uptake in all areas. balance the ph, and feed with a good chemical based flowering fertilizer. the reason i say chemical, is because it will be readily available to the plant, and wont have to be broken down like some organic ferts.