It's explicitly mentioned that the reason it's a hard check is that you only have occasional or partial footprints.Firm Ground
Most normal outdoor surfaces (such as lawns, fields, woods, and the like) or exceptionally soft or dirty indoor surfaces (thick rugs and very dirty or dusty floors). The creature might leave some traces (broken branches or tufts of hair), but it leaves only occasional or partial footprints.
Not partial tracks or occasional footprints. The likely reason is that the long grass bent very easily. Long grass bends and breaks in the middle, and holds tracks for a long time. That also makes it easier to see where a few hobbits ran off.for he had seen footprints that went that way, branching off from the others, the
marks of small unshod feet.
It's long grass as Aragorn sang around p 4.
Through Rohan over fen and field where the long grass grows
The West Wind comes walking, and about the walls it goes.