Can you give us some more information?:
What strain is it?
How long so far in Veg?
What size pot?
Size of grow space (LxWxH)?
Make and model of grow light?
Humidity % in the environment?
PPM of medium?
How often are you watering?
i use Fox Farm Nutrients several years now - and when i first started with the main 3 Fox Farm Nutrients i still had no major idea about PH Balance or tester but i did well with some setbacks but not like pictured below the ugly one lol
the next thing was going through alot of PH Up after mixing Nutrients - found that adding after was a waste of solution trying to hit target range - best is add 1 to 2 teaspoons before any Nutrients - still have pretty much a full gallon of UP for about the last year and a half
PH does not seem to be the factor but the single Nutrient just might be the cause as Big Bloom seems to be the base line food the entire grow as it does not have the other essential Nutrients needed by the plant
1 Big Bloom
2 Grow Big
3 Tiger Bloom
had very good results with just these three - now just feeding only 1 nutrient Big Bloom after the first couple of weeks might or can cause the problem you have now depending on how old they are - 7- 14 days or 21 to 35 days
the next several weeks the plants do need the next nutrients in schedule as listed if you have the feeding schedule as well as the other two nutrients - if not its all guess work trying to figure out if more Calcium is needed or Magnesium and so on its not fun trying to guess
and if you do not have Sledge Hammer for the flush what i used was Molasses - and trying to keep my PH at or about 5.9 to 6.2 - in that range i had a very good grow with a good size harvest
feeding only one nutrient if not any at all but guessing what it needs this is the result - never made it to bloom or full flower just turned and withered away
then getting the right schedule and amounts this is the next grow
changed locations and made new chamber for the grow - as you can see what a difference once you get things in a good balance
well that ugly plant ended up becoming mulch for the soil
i do not remember if it was one of Chris's videos that talked about the uptake of nutrients by the roots for the plants - and possible causes for root lockout of certain nutrients - give him the info he asked for as it can help alot to what might be taking place and correcting it early on
i do hope some of my personal findings of Fox Farm Nutrients helps you out in your search for help and possible answers