My American Standard Plebe looses water in the toilet bowl. I must push the handle, wait until the bowl is full of water and flush a second time. What could that be caused by?

Other than a cracked bowl the only way for water to be lost is by siphoning due to a bad vent stack. Does the bowl fill completely after flushing and then slowly lose water oevr the next few minutes? More detail may help us figure out what's going on.