At first glimpse I'd lean at potassium deficiency, but as @Cap Stone stated more info is needed. Whenever there's a deficiency my first check is the condition of the soil, ph-ppm-ec. Any or all of those can cause issues whether there is the right amount put in or not. If it's possible for you to check those it would greatly increase the odds we can help you out better.
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At first glimpse I'd lean at potassium deficiency, but as @Cap Stone stated more info is needed. Whenever there's a deficiency my first check is the condition of the soil, ph-ppm-ec. Any or all of those can cause issues whether there is the right amount put in or not. If it's possible for you to check those it would greatly increase the odds we can help you out better.