Is you toilet flange sitting flush with the floor? The rim of the flange should be level and sitting flat on the floor with no gap under it at all. If not, that is the reason. Some toilets have a larger area to fit incorrectly installed flanges. You need to ensure that the toilet is shimmed so it doesn't rock for the wax seal to work. Also, the floor may not be flat. While possible, it is not likely that the toilet is defective.