nitrogen, replished or change soil, trim roots.
i grow bonsai mums. read an article about it a few years ago. i flower 20 at a time so i'm not getting big #'s. you leave your plant low in the pot. as they grow you slowy fill in around the stalk. take cuttings until stalks are solid and pot is full. when it gets root bound, you take your cuttings, remove plant from pot, trim a couple inches off outer root ball. they used a gensue knife and laid it down and cut it into a square block. trimming off 2" from sides and bottom. replant with fresh soil allowing room to add soil to continue to bury main stalk. eventually you've buried it so far it branches off into several plants. it is said you can cut these down the middle and pot individually.
so to make a long answer short: nitrogen.