Why is there a septic/methane smell only in our kitchen?
A stutgard? I assume this is a brand of air admittance valve (AAV)? This is basically a one way air valve that allows air into the vent to break any siphon action on the trap seals, but supposedly prevents sewer gases from coming out of the vent into the house.

All the AAVs I've seen involve a lightly sprung wafer valve mechanism. They can sometimes become wedged slightly open due to debris or poor fit. Usually manually operating the wafer will free it so that it works normally again. You should be able to access the AAV itself, they are supposed to be installed in an accessible location for this reason. You may need to improvise a tool to manipulate the actual valve mechanism.

These devices have never struck me as completely gas tight when closed, it's not hard to imagine some small amount of gas could filter through enough that someone sensitive to such odors might detect it even when the AAV is operating normally. This last bit is strictly my opinion and disagreement from others is entirely possible.