you can over fert it too
it could be needing a flushing from salts biulding up
it could be getting root bound
not enough water
or too much

i suggest go organic,(natural fertilizers,they are hard to give too much of since it will only take what it needs ,naturally
you can buy bags of cow manure mix half and half to soil and it wont need anything but water,most people screw up with ferts trying to rush to big huge buds,
natural is best safest way till you know what your doing.
good luck hope it turns out for you