Do you see the brown spots on the underside of your leaf too? It could be from too much nutrients. If you don't see the spots on the underside, it might be a calcium or magnesium deficiency. I had this with my plants as well; I'm now giving them calmag along with my nutrients, 1.5 ml in 2 liters of water at pH 6.5. The spots obviously won't disappear afterward, but the plants are still blooming well and haven't been too affected by it, and I had that problem when starting and throughout the entire bloom. Feel free to check my diary. Cheers!