1st I don't have the problem with the DoT fall off, They both continue ticking after the effect of Soul Harvest. (I have Crippling Death)
2nd some monster seems to take more damage by this effect than other monsters for some reason (maybe It's a bug?).
But for me, they always take damage.
3rd It's work fine with any kind of monster, Goblins including.
Maybe the thing you should consider is the area of effect of Soul Harvest and your latency.
Soul Harvest has a bit narrow AoE (12 yard I think).
With some monster that always running around like a tiny mobs or Goblin and a bit of latency gap can cause them to out-range your Soul Harvest area before the skill resolve.