I see this when I am hitting my plants to hard with nutes in flower... usually like around wk 3 or 4..
That said... often these kinds of problems are a result of something not exactly related at first glance or without knowledge of the elements..
Like, excess of one nutrient causes a lockout of another... maybe it is A def due to excess B causing lockout of A..
Give us some more info...
media
nutes
feeding/watering sched
veg?flower? week?
If you are in soilless.. the usual easy fix is a check of the runoff... flush with appropriate ph (related to runoff), then water withcompletenute.
Look for healthy NEW growth... old growth does not "fix"... except N yellowing