Just bought house built in 1999. Master bath has two lavatories, shower and toilet. No vent stack in attic. A mini studor under each lavatory mounted above the tin line of the horizontal drain from sink. All drains work well except toilet. Toilet work great with studor removed so replaced the studor vents with new ones. No help. When toilet is flushed with.out studor in place, bowl empties fast which leads me to believe there's no blockage down stream, however I have not snaked it more than 3 feet with a closet auger. Any thoughts?