Only happends last few weeks of flower

So grew same stain last time, and same thing happens. End of leaves dry up and whole leaf starts turning brown and this starts three or little more before done also buds tips are weird and airy some buds look more dence than other can't figure out friend said cal mag on last grow gave more same problem. Please help

 

Oh and happens more than few weeks in just start two weeks of flushing now

 

70 to 75 not hot at all lights are far as well they die and curl up not down