Why is the toilet bowl water level is low?

The water level in a Toilet Bowl is determined by the height of the outlet of its internal P-Trap.
When the water level in the bowl is too high, it is usually the drain pipes are clogged.

But when it is too low, there are at least 4 possible reasons:

1. A partial clog that slowly siphons water from the bowl.
2. Insufficient water in the tank.
3. Blocked vent pipes.
4. Hairline crack in the bowl trap way

1. A partial clog that slowly siphons water from the bowl:
   Use an auger to clear the trap way (use an auger with a bulb head).
   
    
2. Insufficient water inside the tank:
   Remove the tank cover (lid) and check the water level after the fill valve has shut off.
   It should be about 1" below the opening of the overflow tube.
   If it is significantly lower than that, there may not be enough water in the tank to fill the bowl after a flush.
   **Correct this by adjusting the fill valve float.**
    
3. Block Vent Pipes:
   Listen for gurgling sounds from the toilet when you use another toilet or empty the bathtub.
   There is an indication that the plumbing vents are blocked and the vacuum in the pipes is sucking air through the toilet and probably emptying the bowl.
   **Clear the vents or ask a plumber to help you.**
    
4. Hairline Crack in the bowl:
   Look for water on the floor around the base of the toilet if you cannot pinpoint any other reason for the bowl's low water level.
   You may have to remove the toilet to get a good look at the subfloor, especially if the floor is tiled. The presence of water probably means there is a crack in the bowl.
   **If so, replace the toilet bowl**