It looks like nutrient burn from nutrient salt build up, to me.
I assume you are using 12-4-8 Liquid All Purpose plant Food, or maybe 8-7-6 Liquid Miracle Gro Plant Food Drops. Either way these are really high in nitrogen and should not be used for the entire grow. Cactus plant drops 2-7-7, or their tomato plant food 18-18-21 have better NPK ratios to what cannabis needs.
You can use miracle gro but then you do risk running into problems easier, they use a lot of a type of nitrogen that can cause overly acidic conditions and pH will certainly be harder to keep stable in some instances.
You should probably be watching your nutrient salt concentrations in your soil or in the run off. You likely have an over abundance of nitrogen salts built up, along with potentially a lot of other mineral salt build up – that could very well be contributing to this problem. You can check the run off of your soil with EC/TDS/PPM meter. You can buy these easily enough on e-bay, amazon, etc. Or you can get a simple soil probe “spike/rod” kinda thing, that does more than just moisture and pH readings. One that measures nutrient concentration or “Soil Fertility” Something like this:
In the meantime flush the soil with 3 times its volume of water and then feed using the cactus drops as per directions on the bottle or use the tomato plant food at one tsp. per gallon, if you don’t have easy access to a better balanced nutrient system, or better yet use our Marijuana Booster nutrients.