They like a humus rich soil in light shade. They are beginning to self seed here, even in the very dry parts of the garden. My soil is an acid clay with not as much leaf mould in it as I would like. When I planted the corms I did put leaf mould in the base of the hole.
In the past I did grow them in pots of MPC before planting them out, try that.

as soon as you get the corms, soak them overnight and then plant where you want them to flower. Or into pots of compost and then plant when they are in leaf come spring. I do the latter as I find they settle into the soil.

as soon as you get the corms, soak them overnight and then plant them where you want them to flower. or grow in pots and then plant out in spring when they are in leaf. that's the way I do it, but soaking does make a big difference.

Thank you Owdboggy and Seaburngirl. I will try again, following your advice.