If the water level is too low to cover the plunger cup, you won't create the necessary suction.

It is a common and frustrating scenario: you’ve been plunging for 20 minutes, your arms are tired, but the water level in the toilet bowl hasn’t budged. When a plunger fails to clear a clog, it usually means the blockage is too dense, located too deep in the pipes, or you’re using the wrong tool for the job.