What causes low water level in toilet bowl. The bowl will will fill to normal level when flushed, the 30 minutes later is is very low.

Tom, Speedball1, can give you an explanation regarding defective china. It can also be caused by a clogged vent stack that siphons water down the drain. If it is a clogged vent it would drain at different rates almost every time. A defective china would drain identically each time.