Over the past few months I have been asymptotically approaching being able to make a good cappuccino or latte. (I'm way better than the glassy-eyed morons who are usually behind an espresso machine but nowhere near "good barista".) However, over the last few weeks I have noticed that sometimes I was not able to steam the milk properly. I get very little foam then when I bang out the biggest of the bubbles, it’s virtually flat.
I had thought that I was just having a bad run, but then a colleague mentioned it too. It turns out that only some of the bottles have this characteristic, and we haven't determined, from analysis of the codes on the bottle tops, what determines this. Each was full-fat milk from the same dairy.
If anyone who reads this knows the answer, please mention it in a comment.