non flowering snow drops the most common reason is that they were cut back to soon after flowering last year. Each year after flowering the bulbs grow the flower for the next season inside the existing bulb so if they are cut back to soon after flowering they do not have time to do this.To solve this I would feed the area they are in with a general feed such as growmore fertilizer and then allow to die back naturally then next year they should flower.

Also sometimes when flower drops become to dense this can create non flowering. So after a while its a good idea to split and move them the best time to do this whilst they are in early leaf.