Why Some Counties Have No Snow Load Number
Two different reasons, one blank cell
A county with no design load on SnowLoadCheck falls into one of two categories, and they mean opposite things. A Case Study area is the map being accurate: ASCE 7-22 §7.2 itself declines to give a single contour figure where mountain terrain changes elevation too fast for one county-wide number to mean anything. Not returned by the map is a real data gap: the map query came back with no feature at all at that county's reference point, almost always a shoreline county whose point sits just outside every mapped polygon.
What a Case Study area looks like
King County, WA is a Case Study area: it spans sea level in Seattle to alpine terrain in the Cascades within one county boundary, and ASCE 7-22 requires a site-specific elevation-based study rather than publishing one figure that would be wrong for most of the county. This is common across the mountain West - 386 US counties get the same treatment nationally.
What a map-query miss looks like
San Francisco County, CA is the other case: a small, dense, coastal county whose surveyed reference point falls just outside the map's polygon grid. Unlike a Case Study area, this is not the code making a deliberate call - the real ASCE 7 value for this location is almost certainly available a few hundred feet away, and the free ASCE Hazard Tool will return it from an exact address even where the county-level query misses.
What to do with either one
Neither case means "assume zero." Both mean the same thing this site says everywhere else, more emphatically: go to the ASCE 7 Hazard Tool and enter the exact address. It handles elevation-specific and edge-of-polygon cases directly, which a county-level lookup by design cannot.
Where this plays out
See the range across real counties: Aroostook County, ME, Edgecombe County, NC, Kalawao County, HI. Related: Fresh Snow, Packed Snow and Ice: Why the Same Weight Looks So Different, Snow Is Already on the Roof: What Actually Matters.