They are not yet in the flowering phase, am I seeing that right? They desperately need nitrogen. Is your fertilizer a special product for cannabis or just regular fertilizer? Hemp is a very fast growing plant as you can see and they need LOTS of nitrogen during growing stage.
Once they are in the flowering phase they won't be able to get nitrogen from the roots, instead they will use the leaves as reservoir. Make sure you have a very good bloom-fertilizer once they switch to flowering.