It is important to know if you are adjusting the pH immediately before pouring it on the soil, with everything mixed in. This pH should be 6.3, every time.
So the difference between the plants is probably your fault, with some getting feed and some not... and things get curiouser with the results of that experiment.
FF is a scientifically designed system that works very well when you follow their extensive instructions. The scientists that designed the feeding chart know that our plant needs different nutrients at different stages of its growth. To this end, the dosage and mix of the trio changes from week to week. This isn't optional with this grow system. For all the things to work well together, you really have to follow the program. I strongly recommend downloading the soil feeding chart and then following it religiously from now on. You are going to find that you are not only giving the wrong combination of these nutes right now, but you are not giving enough. The dosages suggested on the bottles are not for precise indoor work, they are what you go by in an outside garden.
Your biggest problem though is most likely because of a buildup of salt in your soil, and this is blocking some of the nutrients. As FF breaks down, it releases a lot of salt into the soil, and this needs to be flushed out. If you don't flush, you are going to start having some big problems toward the end of the grow. 3x the container size in one watering session is how to do it. Reset the soil and then it will work well for you again.
This also brings to mind another item on the feeding chart. They suggest that you feed your plants up to two times a week. In soil, the common and preferred way of doing this is by following a feed/water/feed/water schedule, providing plain pH adjusted (6.3) water inbetween each feeding. This does several beneficial things, but by feeding every other time you can keep the soil a lot more clean. By following the suggested dosages in the feeding chart, half of the feed that you give is temporarily stored in the soil, so that it can be used on the water only pass. Soil is a great growing system, but you have to use the tool correctly to get maximum benefit.
Here is the feeding chart: