i can definitely without a show of a doubt tell u you bigger ones are showing some signs of slight heat stress, which may be part of the problem. on the other hand it may just be a runt, like was already mentioned. if you are able to correct the heat situation and your PH is fine and there's no improvement, u just got a runt. get the heat straightened out now or else you will regret it down the line, ive struggled with some minor heat issues this summer myself. see how all of the leaf edges are turned up slightly at the edges as well as the brown tips