A common reason for a falling water level in a toilet bowl is a clogged vent. The water passing through the pipes creates a vacuum that pulls water into the drain from the bowl. Other reasons for dropping water levels include improperly installed drains and hairline cracks in the porcelain.
Drain systems move wastewater with gravity. Vents extend from the drainpipes through the walls and ceiling of the home to allow air to help gravity push the water through the pipes. Animal nests or other debris sometimes clog the vents, decreasing or stopping the flow of air. Pushing a plumber's snake through the vent usually removes the obstruction and stops the toilet bowl from losing water.
In order to operate correctly, the drain should slope gently away from the fixture. Due to improper installation or settling of the home, some pipes slope toward the fixture. This results in water collecting in the pipe under the fixture, which also pulls water out of the bowl. Fixing this problem requires adjusting the line so it slopes correctly.
Porcelain sometimes develops hairline cracks. While some of these cracks allow the water to leak from the fixture onto the floor, others affect the toilet by allowing water to leak directly into the drain. If the vent is clear and the line slopes correctly, a porcelain crack is the likely cause of losing water in a toilet bowl. Repairing this problem requires replacement of the toilet.