Are they in a poorly drained area? If too wet the bulbs can rot. Do you have good air circulation? What is your soil like?

They are on a gentle slope with amended soil (originally clay), and I think the drainage is good. Some seem fine, the rest are slowly dying...

Could it be a deficiency? I don't see any bugs or beetles anywhere.

Might be a basal rot... Can you dig up one of the bulb and examine it? If they do have basal rot, you can save some scales at least.

I'll dig some up and check them tomorrow. I sure hope I can save them.