Most toilet bowls retain water once filled. The inner workings of the tank moderates how much water is sent to the bowl and in some cases it will affect how much water stays in the bowl. There are several other factors that affect the water level in your toilet's bowl. The problem may not reside with the toilet at all.
The most common issue that causes water loss in a toilet bowl is a damaged or old flapper valve. This is the valve at the bottom of the tank, and it is connected to the handle by a chain. When the toilet is flushed, the valve opens and water drains from the tank. If your toilet bowl loses water, replace this part. Simply pull off the old one, remove the chain hook, then insert a new one and reconnect it to the chain.
Water Level Is High
In some cases the water level in your bowl may be too high, or too low, for the toilet to function properly. This causes water to improperly drain from the tank. To adjust the water level, simply tighten the drain valve with a screwdriver. The drain valve is located in the toilet tank, on top of (or at the bottom of, depending on the model) your toilet's drain pipe. Adjust the water level to increase the amount of water in your bowl.
Broken Bowl Ring Seal
The toilet is connected to the home plumbing through a flange in the floor. At the base of the toilet there is a wax bowl ring that acts as a water-tight seal between the toilet and this flange. If the toilet has been shifted, or jarred in any way, this seal will break. The result will be a loss of water from the bowl. Check the floor for signs of water leaking from under the base of the toilet. Water stains on walls or floors near the toilet's base are another sign that this is the reason the toilet is losing water.
A slightly less common reason toilet bowls lose water is due to improper venting. This is an issue with your home's plumbing and there is no DIY fix. You will have to call a plumber to solve this one. If your pipes are not vented correctly, waste passing by the toilet pipe will create suction and pull water out of the bowl, lowering the water level inside.
The final reason a toilet loses water is through a damaged toilet. A crack inside the bowl, or at the base of the tank will cause a leak. That results in a lower toilet bowl water level. To check for this problem, inspect the bowl for hairline cracks and damage. Water stains or water at the base of the toilet is also a sign that this is the problem.