Helleborus niger is a bit tricker to grow than the other common types found at retailers. Of all the commonly planted varieties, niger is the most sensitive to soil pH, preferring a more neutral or even alkaline pH and I think you are right to assume they do not like your growing conditions. Unfortunately, liming to reduce soil acidity is not a very fast process - it takes about 6 months before the effects are recognized - and the suggestion is to make sure you have adjusted pH well before planting.

I'm not sure you can do much more than you are doing now. Mulching the plants with limestone chips is often suggested and could be beneficial. Make sure watering is not excessive and that where they are planted, the soil drains freely. That's all the advice I have to offer - good luck!

Thank you very much for the advice. Sadly the one place I've got that I could turn alkaline or neutral, doesn't have the best drainage. I'll try the limestone chips and hope for the best.