OK, what I see happening here is a calcium deficiency, but you shouldn't have this based on the nute system and soil you are running. The reason that you are having this seems to be multifold.
First, flushing needs to be understood because in a FF grow, flushing is called for several times on the feeding schedule. This is not a joke and they are not simply trying to sell you their flushing agent, Sledgehammer, they know that their nutes will back up and cause lockouts if not flushed away periodically. A flush, without Sledgehammer is defined as pouring 3x your container size in fresh water through your soil, in one sitting. A flush with Sledgehammer will require less water. The goal of this is to flush or WASH any leftover nutes and built up salts from the use of those nutes.
A watering with plain pH adjusted water has not ever been known as a flush before the bro-scientists and keyboard gurus started to change the meaning of the term to include simple watering, but in the agricultural world, flushing has always meant flushing the soil so as to clean it out. FF clearly shows that you need to flush between weeks 4 and 5 when you switch away from purely veg nute mixes and then twice more before end of the grow. If you do not proplerly flush a Fox Farm grow, you are very likely to have problems.
It gets worse though... you are using a F/F/W schedule (feed/feed/water) instead of the traditional F/W/F/W schedule in soil. There is a good reason for alternating feed and water. The dosages given in the feeding chart are actually enough for two feedings, but you give it once and then give water the next time, properly pH adjusted so that you activate the nutes that are temporarily stored in your soil. This is the problem, the soil is designed to capture half of the nutes and feed the rest to the plants, and when you give plain water the next time it not only cleans out the soil of any leftover nutes, but it also effectively feeds the plant a second time.
So you are overfeeding the first time, really overfeeding the second time, and then the third time you allow the plant to use up some of the stored up nutes in the soil. Not only are you wasting nutes, but you are also not at all cleaning out the soil as the system was meant to be used. You would get much better response from your nutes if you spread them out by doing a plain water inbetween.
You very likely have a buildup of both salt and nutrients in your soil. This is causing a lockout or an inability to get to calcium some of the time, hence the slight deficiency. Do a 3x flush and treat that as a normal water only day. There is no reason to pH adjust this water because you are flushing away the nutes, and it is only for the nutes that we bother to pH adjust anyway. After this, alternate feed and water through the rest of the grow, and watch for your next call to flush.... especially at the 6 week point, as the buds are going into their final buildup, don't forget to flush so you can clear the pipes for full nutrient uptake for a spectacular finish.
And no, I did not recommend flushing immediately before harvest, or starving the plants of nutrients during the last week or two.... just continue feeding them and 3x flushing when FF calls for it.
