sometimes there is nothing you are doing wrong it is just a weak plant. remember we are dealing with a living entity and it is always in gods hand weather it lives or dies. we do our best to give the plant the best quality of life we can but there are some things we just cant see and just will never understand. i have had plants get 8-12 weeks old and just go belly up while other plants have doubled in size in the same system. it is a law of nature. do your best with the remaining 10 and enjoy your grow.