Make sure the small tube from the ballcock is still putting water into the overflow tube of the flush valve--this adds water to the bowl while the ballcock fills the tank.
: The water level in the tank is okay, but the two upstairs toilets have only a couple of inches of water in the bowl, while the downstairs toilet bowl has a normal water level. The vent stack seems to be unobstructed. This was confirmed by running a garden hose down the vent (no water, just the hose) to check for any obstructions. The toilets flush normally. It's just that they don't have enough water in them after the cycle is complete.
: Any advice?