Partial Flushing Due to Low Water Flow
To flush properly, a toilet must fill the waste line with water quickly enough for suction to develop and pull the water out of the bowl. A water-saving toilet may not supply enough water, especially if it replaces an existing "water waster" that was working properly. The pipes may have been appropriate for the older toilet but may be too large to be sealed by the smaller volume of water. The result is a partial flush that doesn't clear the bowl. This problem can be corrected by replumbing the waste lines or installing a jet-assisted toilet.
Partial Flushing Due to Insufficient Water Pressure
Even a jet-assisted toilet doesn't always work the way it should, and the problem is usually low water pressure. To compensate for the small volume of water, a jet valve opens when the toilet is flushed and sprays water into the bowl under pressure. This valve can only work correctly if there is sufficient water pressure, though. If the house has undersized or corroded plumbing pipes or a low-pressure source of water, there isn't enough water available to create suction and the result is again a partial flush.
Poor suction caused by low water flow can cause a related problem. Because the flush does not completely clear the bowl, sediment collects in the bottom of the toilet's internal P-trap and restricts the flow even more, eventually clogging the toilet altogether. This is not usually a problem with jet-assisted toilets, because the pressure from the jets eventually moves some of the sediment along. Clogging was a frequent problem in first-generation water-saving toilets, but as toilet technicians have tweaked inner toilet dimensions to compensate for smaller water flow and to produce better suction, later models have become more reliable.
Users of water-saving toilets frequent complain that the low water flow causes debris to collect in the waste lines and damage them. This isn't a problem caused by poor suction, but a direct result of the smaller volume of water moving through the pipes. It may be particularly troublesome in older houses with cast iron waste lines that have enough internal corrosion to collect sediment and allow it to build up. Alleviation of the problem isn't always quick or easy. It sometimes involves retrofitting the waste lines so they will clear with a smaller volume of water.