I recently picked up a Londinium R as an upgrade to a Breville Dual Boiler, and around the same time I started buying grass-fed, non-homogenized milk. I was getting frustrated that my microfoam had a lot of little bubbles in it, and had been assuming this was due to the vastly increased steam power from the Londinium vs. the BDB. It was pourable, but bubbly and not proper microfoam.
Well, yesterday I decided to go back to regular (homogenized) 2% milk, and the results were perfect. No bubbles! Of course I blew the pour and drew a pretty wilted tulip, but at least I figured out the milk was the issue. Likely the grass-fed aspect doesn't matter, but homogenization does. At least, that'd be my assumption. So fat distribution might be important.
So, if you are having trouble getting microfoam despite your best efforts, try some different milk!