How to vent a new toilet without a vent stack?

[RHiggens]

I recently built a bath house and though I tried to plan ahead for everything I completely left out a vent stack in my plumbing plan. I realized this AFTER the roofing was done and the drywall was up! I tried to fix my mistake with studor vents. One under the sink vanity, and another on the line the collects the shower and washer. The toilet started flushing slow after working fine for several months. I added another studor vent by extending the cleanout up under the house (it's open under the house 48" to the ground). But nothing changed. When I unscrew the vent the toilet flushes fine. Any ideas on how to resolve this without make holes in the walls/roof of my new bath house? Thanks...
Rick

 

[TicklyT]

If it worked once, but has now stopped working, then it sounds like a possible partial blockage.
Air admittance valves do not allow positive pressure in drains to dissipate, they only operate if the pressure in the drain falls below atmospheric to prevent water being syphoned from appliance traps.
Positive pressure is usually the result of a blockage somewhere.

Has any of the drain pipework sagged or dropped since installation?
Is there sufficient 'fall' on the drain run?

UK practise does not usually require an open vent on a stub stack providing the fall from the appliance(s) to the drain invert is less than 1.5 Metres, the stack is not at the head of the run and there is an open vent elsewhere.

US building codes and practises differ from the UK, so I can't comment on whether an open vent stack is necessary in your circumstances.