soil im guessing. ur soil probably doesnt have a whole lot of nutrients left in it. id go with a decent micro and bloom nutrient and add them a little at a time. maybe 3 or 4 ml per gal of each to begin with.
soil will only hold nutrients in it for so long. the bigger the plant and the smaller the container speeds the process up. next time try a little bigger pot size. not bad at all tho brotha keep up the good work.