Dear researchers, has anyone encountered this phenomenon before?

Several times when I perform pure gas permeation test for CO2/CH4 on pristine PPOdm membrane, the flow rate of CH4 is faster than CO2. Could the transport phenomenon be attributed to other types instead of solution diffusion in this case? How to avoid it or could you share your experiences in conducting gas permeation test?

I am conducting it in a simple bubble flowmeter.