top leave necrosis during second wk of flower.

Running into several alarming issues w the new auto flower grow. Havent seen it and will do shortly but have been told by partner grower that we have an issue arising. Will post a pic of actual issue later this evening. But here is my description.
Brown dying/dead leaves around the buds. Plant info is as follows.
Afghan skunk x 2
Auto cookies x 2
Auto northern light x 1
All plants 5 wk from seed 1 wk into flower
Lights
400 w hps running at 50%
250 w led
Dirt 1 gallon bags
Fox farm happy frog
Nutes
Fox farm line
grow big
bio bloom
tiger bloom
been feeding 1/3 -1/4 strength.
1st feed veg nutes 1/4 strength
2nd feeding 1/4 strentgh bio bloom
Lights switched from metal halide during veg to hps when flowering began. Hps was introduced at 100% power then dropped to 75%. Im not thinking light burn as lights are 20+" away.
Plants have exhibited tremendous foliage out put perfect stalk production during both veg and flower.
Im of the opinion that there is a lockout that i hope can be rectified w epsom salt. Will do 1/4 tsp per gallon

Why do you have the 400W hps at 50%? Crank it, plants need light to use the nutrients you give them. If you have the 400W hps in a hood with a glass lens and fans blowing across the canopy it can be with a few inches of the plants at full power. Pictures will help others help you though.