It looks like you aren't using enough CO2, just for starters. Even with surface movement you should get a bigger drop in pH with the CO2. Have you tried figuring out how much CO2 is dissolved in the water?

Another idea is to use one of the in-tank pH measuring devices, converted to a CO2 measuring device. For details, see: DIY Drop Checker - Aquatic Plant Central- aquascaping...a living art