Upper Leaves all turning yellow and wilting

Hi there,

I have four random plants planted in a storage container in hydroton and lava rock. They are top fed and drain freely to another storage container below it. I'm 21 days into flowering and the three other plants are doing great except this one. The upper leaves are evenly turning yellow and wilting but the buds appear to continue to develop. Maybe this is just what this plant does when it buds? I'm not sure what to make of it but it doesn't appear to be overfertilization, overwatering or pests. Please Help!

sounds like a nitrogen problem, are the buds about ready to be harvested yet or do they have a ways to go.

could be way too hot, how far are they into flower?