What is the likely fix for a pump that won't prime?

Check the pump lid o-ring first. Air getting in kills prime, and that tiny rubber ring is the most common leak point. Remove it, clean both the ring and groove, then apply silicone lubricant (not petroleum jelly). Hand-tighten the lid only. If that doesn’t work, check the drain plug, but 9 times out of 10, it’s the lid o-ring.