If you are talking about water in the bowl being low, it can be caused by a couple of things. The first is the fill tube from the float valve to the overflow tube. If the fill tube is not pointing into the overflow tube and refilling the bowl after every flush your bowl will not have very much water in it. Each time the toilet flushes, a siphon takes place that sucks the toilet paper and liquid down the drain. The siphon is only broken by the air that is drawn in after the bowl is empty of water. Water flows from the float valve (fill valve) through the fill tube above the water level in the tank and goes down the overflow tube, bypassing the flapper, refilling the bowl. If there is no water then the bowl does not refill. Check the tube to see if it is attached 1" above the overflow tube and there is a stream of water going down the overflow tube.
If you have a clogged vent, you may also have a problem with a toilet bowl not being full. Each time one of the other toilets flushes, it draws water out of the full toilets by vacuum. That vacuum is normally relieved by drawing in air from above the roof through a vent system. For every gallon of water you put into a plumbing system, you have to push a gallon of air. As the water flows past other fixtures, it has to draw air in behind it to replace the space. This creates a negative pressure, which is relieved through the venting system, unless it is clogged.