I would first ask if they are in a dry area? Daylilies love water. Have they ever been fertilized? Those are the first two things that come to mind, even though I have never heard of daylilies not blooming over such a long period of time. So I would suggest lots of water and some balanced fertilizer to start with, of course a soil test is always advised when you have such problems.
Edited to add:
If they have been there for 10 years and had no blooms, they might benefit for digging them all up, adding compost and other amendments to the the soil, thinning them out and replanting them.
