We have a modular home with air admittance valves under all the sinks and one vent pipe through the roof. The toliets were not flushing as they should. I discovered that they would flush when any one of the air admittance valves were removed, so I replaced the air admittance valves under all the sinks. The toliets seemed to work a little better for a day or two, but are now back to not flushing everytime. I checked the pipe in the roof and there is some type of flap or something in it. Could the problem be with the roof vent? Thanks for any suggestions.