"I have toggled a layer to be ‘visible’ but it is not showing on the map. Why not?"
Certain layers within the maps are "scale dependent". These are generally complex layers that are too detailed to display well at smaller scales (i.e., when viewing large areas). For example, if a "Tax Parcels" layer were turned on and the user were zoomed out to the full extent of Tompkins County, the amount of data required to generate a new map image would be immense. To prevent the map image from becoming too dense cartographically, and to enhance system performance by reducing the amount of data being processed, some layers have been assigned a fixed threshold that determines at what scale they will appear.
Generally, a scale dependent layer will only be drawn when the map is zoomed in to a large enough scale. In a few cases layers may be scale dependent in the other direction -- they may appear at small scales but disappear when the map is zoomed in.