All plumbing follows the basic laws of nature... gravity, pressure (or venting), and water that doesn't seek its own level. Having this in mind will help in keeping an understanding when challenged by the plumbing demons.
First, water supply systems and water drainage systems must always be two distinct subsystems of plumbing as opposed to drainage and venting (the sewer system). Fixtures (plumbing bridges) bridge these two systems together. Air admitted by the venting of pipes keeps the water flowing while averting siphoning out other traps, keeping them sealed, and preventing sewer gases from entering the structure through the drains.
Despite what many people believe, like supply systems, drainage system performance DOES depend on pressure; or the absence of negative pressure. Waste water (matter) drains through the pipe due to positive flow, as in drain line pitch or angle in a downward direction; gravity. Sounds simple enough, but there's more to it... i.e. venting. Vents allow air to enter the drainpipes when negative pressure is generated from the flow of fluids through a pipe. to proper performance. This is called the Bernoulli Principle or the Bernoulli Effect: In fluid dynamics, Bernoulli's principle states that for an increase flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. To put it another way, an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.
Most of the time when we think of pressure we thing of positive pressure however, pressure can be, negative or positive. For example, plumbing vents allow air to enter the drainpipes decreasing pressure; negative pressure. If there were no air supply coming from the vents, wastewater would not flow out properly and the water would be slow to flow or traps could potential be siphoned away. Think of your finger over the end of a straw filled with fluid. Thus, drainage and venting must work simultaneously.
Hydrostatic perpetual motion? Perhaps. In whatever the case, all fixtures must have a trap, and ALL traps must have a vent nearby. This does not mean you can't tie all together to one venting system or use individual AAV's, but ALL must vent after the trap prior to another trap or any long, (especially straight downward) runs that create pressure, albeit negative pressure.
In your case, you say the AAV is not opening... that is because there is not enough pressure to open it but enough to effect fluid flow performance. Check the DFU values in your system per fixture as these values become critical when combining open venting systems with mechanical devices which relys on pressure to function properly.
Disclaimer: This post is a matter of opinion; I am not a licensed plumber nor an aeronautical engineer moreover, just an average Joe DIY'er somewhere in between who ignores California Proposition Acts while learning fearfully through trial and error; maybe it's just a Texas thing.
