Not sure of what it is. Are you useing a good soil are you useing Nutes Last, I would flush and then if not useing a good soil I would transplant a day after the flush in good soil Fox farm/ roots organic I would use some veg nutes
Try to give as much info as possible. It could be nute burn, heat stress, bugs etc. Try going with the most obvious things and rule them out one at a time. It doesn't look that bad so I wouldn't worry too much. The burn spots aside, your plants look pretty healthy. Good luck my friend.