What is the most common cause of a skimmer losing prime?

The pump lid o-ring is almost always the culprit. Air gets in on the suction side, and the o-ring dries out, cracks, or gets debris under it. Clean it, lubricate with silicone pool lube, and reseat it — or replace it if it’s hard or cracked. Check water level, weir door, and skimmer basket first, but nine times out of ten, the lid o-ring is the fix.