A toilet bowl that only fills up partially won't flush properly, which can be frustrating. Further, partial filling is a sign that there may be bigger plumbing issues that need to be repaired.
Fill Assembly Issues
Most low water problems are a result of problems with the fill assembly, which is located in the tank of your toilet. This assembly consists of the fill tube, which is a flexible hose, the flapper and flapper arm, and the float.
If the fill tube becomes damaged or disconnects from the water source, then the system will cut off before sufficient water — or any water at all — flows into the toilet bowl. The tube must be reattached or replaced. As for the flapper, if it is closing too rapidly, usually due to a damaged or poorly aligned float, water flow may also stop before the toilet has a chance to fill the bowl properly. In this case, the float needs to be replaced or realigned so that the flapper closes only after the bowl is done filling.
Damages to the toilet bowl can also lead to low levels. The problem is usually either a leak in the bowl or issues with the emitters under the bowl rim that allow water to flow into the toilet. When the bowl is cracked, the water level often begins at normal level right after the bowl finishes filling, but it drops thereafter. The leak could be a crack in the porcelain bowl itself, or it could be a leak in the waste pipe, particularly where it connects to the toilet.
Emitters usually become blocked by hard water mineral scale buildup, although occasionally it may be dirt and debris from a poorly cleaned bowl. Scrubbing under the rim frequently can help prevent issues. You may also need to clean the emitter holes with a limescale remover if hard water is the culprit.
Every toilet is hooked into a plumbing vent that has an outlet on the roof. The vent removes sewer gases from the toilet line so they don't come up into your home, but it also provides a free flow of air that ensures proper draining down the waste pipe.
If the vent becomes blocked, water won't flow out of the waste pipe properly and the toilet won't flush as it should. This may sometimes lead to partial flushes, which can, in turn, cause lower water levels in the bowl. Vent blockages are typically a result of debris in the vent, so your plumber must clean it out to solve the problem.
Contact a plumbing service if your toilet bowl has consistent low water issues.