How fast are you moving through the field? If you're tearing apart that much and making too much noise, you won't be seeing any bunnies, they'll scatter to the four winds. With snow cover, they know they don't have as much concealment as they might have in the grass, so they'll scatter farther.

Are you using a dog? Shotgun or rifle? How many acres are you looking at? Are you sure the track and sign are fresh?