A ceiling icon is only displayed when at least one cloud layer is reported as broken (bkn) or overcast (ovc). A ceiling icon will not be displayed when cloud layers are reported as scattered (sct), few (few), or clear (clr).
Also, a vertical visibility extending into an obscuration counts as a ceiling (ie: VV003) and causes a ceiling icon to be displayed on the map.
Image 1. Ceiling icon is displayed due to a broken (bkn) cloud layer at 25,000 feet.
Image 2. Ceiling icon is not displayed due to all cloud layers being listed as few.