Very likely true that they are poisoning the soil but the year round shade must be a factor also. I wish you had listed the failed plants. There are plants for zone 4 that may not do well for me. I am thinking of Mahonia repens and the little groundcover Cornus native to northern woodlands.
Have you tried Sanguinaria, Pipsissiwa, Hepatica, Anemone, Lily of the valley (American or the common type, Ferns,Asarum europaeum, Gaultheria, Michellia (partridgeberry)? Have you considered raking up everything to allow for moss to invade? Let us know.
Many of the plants I suggest really take advantage of the early Spring leafless period and then stay green or die down. But some others may do okay with year-round shade.

The phenomena is called allelopathy, a chemical process that a plant uses to keep other plants from growing too close to it. Here in Z5, the black walnut is the most notorious for controlling its space through this "poisoning" of soil adjacent to the tree. Hope that is of at least a little help...

Hemlock the evergreen isn't the same hemlock they make the poison from. Deep shade and acid soil is what's keeping things from growing under it.
Trim the lower branches so light can get under there. Sweeten the soil a bit but not so much you stress the tree. Lot of little zone 4 creepers will grow under it.

Pachysandra, vinca, lily of the valley, partridge berry all grow fine. I'm sure I'll think of others when the snow finally melts here.

I have a row of old tall hemlocks at the new house and so far can grow a variety of shade plants; hosta, pulmonaria, lamiums, bleeding heart. Sweet woodruff didnt' do well. The hemlocks here were limbed up too much, but it does get more filtered light. I'm not a big waterer either, and it's only been two years for this garden. But so far things seem to be thriving and growing well each year. I'll post a link to my photo album, but you'll have to poke around a bit to find the shade garden pics. mixed in. I don't know yet how to post just one picture!

Here is a link that might be useful: garden beds