First grow

so first grow and i know i cant expect much but somehting is wrong here. i got a 70 watt HPS on timer 12/12 a humifier running 20 hours a day and i cant seem to figure out whats wrong. plant is def a sativa and i know they take long till fully mature. the leaves as you'll notice in the photos are yellowing a little, possible burn from flood or fert. Opinions help is much appreciated. i moved the light further away the lower buds look great now very rich green and healthy but the mother bud on top is cranky.

looks like very small heat stress/nute burn, cool them off, and slow down on the nutes, you should be ok