Nute burn or pH issues would be my guess. Nobody will be able to help you, though, unless you give far more details.
How much and how often do you water them? At what pH?
What are you feeding them, how often, and at what dosage?
What are you growing them in? If soil, what kind?
What is the temp & humidity in your tent?
What are you running for lights ? If LED, what is the brand and model?
How close are the lights to the plants?