Without seeing the plants themselves or having a clear understanding of the planting conditions,it is difficult to say with any certainty what might be going on.

Hellebores require well draining soil and some shade (not heavy shade) to perform best. They are not very picky about soil pH but excessively acidic soil can certainly slow their establishment. Once established they are quite drought tolerant.

i would be concerned about possible root competition from the hemlocks. This can make it tricky to get other plants, including hellebores, established as the tree roots suck up all available soil moisture and nutrients. Overwatering to compensate can lead to root rots.

As long as you provide the conditions they prefer, they should be very easy to grow. How to grow hellebores.