We are remodeling a "mother in law" quarters behind our home. The shower will not drain until there is about 3 inches of water in the bottom, and then it rushes out and continues to drain normally.
Other clues: The previous owner had stuffed paper towels into the space above the overflow drain. I don't know why.
There is no roof vent for the shower/tub combo.
The toilets and sinks drain fine.
We have exposed the sewer pipe to the septic tank, and there appears to be a rise of an inch or so as soon as it leaves the house.