I apologize I missed you said they were in midflower on your first post. As budman suggested it appears to be Ca if so and it isn't included in your nutrient line already then find a source for it. Of course this assumes all other things are ok such as you are pH balancing your nutrient solutions and you aren't pushing/dropping the Calcium out with hard tap water or too many nutrients. If you want more help with this then I need a ton more info including nutrients+additives, type of water, how you are pH'ing them and type of soil.
It is always good to check daily for bugs and to know what you are looking for. Spider mites are microscopic they don't actually fly only jump trailing a web string. I've seen severe infestations with strings between plants the spider mites have built. At first you don't even know they are there without looking on the bottom of the leafs in the most shaded areas of your plant. They suck on the veins, people say they are on newer growth but I always seem to find them easiest on healthy bigger fan leaves.
P.S. I can't look closely at pics on this pc but from the little photo i can see it looks like Ca