Coffee – Frothing Milk: Variations by Milk Type

I froth my milk using the steam wand on my espresso machine. With a national-brand organic whole milk, I am able to get a perfect micro foam nearly 100% of the time. However, when I instead use a local, grass-fed, organic whole milk (whose flavor I prefer to the national brand), I am only able to produce a perfect micro foam about 20% of the time. What could be causing this? Could it be differences in the fat content? (They are both labeled "whole".) Does it have something to do with the fact that the national brand was likely from grain/corn-fed cows, whereas the local brand is grass-fed?

Update: The local milk is homogenized and pasteurized. It does not say anything about ultra-pasteurization (i.e., HTST vs. ESL). The "national brand" milk is homogenized and apparently comes in both "pasteurized" and "ultra-pasteurized" forms. I'm not sure which one I usually get, but if I had to guess I'd say it's ultra-pasteurized (i.e., ultra-heat treated) because the national brand usually has a sell-by date at least a week after the local milk's expiration. According to their nutrition labels, both have the exact same nutritional content except:

• Sugar (per cup)
  National: 11g
  Local: 12g
• Cholesterol (per cup)
  National: 30mg
  Local: 35mg
• Vitamin C (% of Daily Value per cup)
  National: 0%
  Local: 4%
• Vitamin A (% of Daily Value per cup)
  National: 4%
  Local: 6%

Protein content is reported as identical.

Update #2: As I mentioned in a comment to TFD's answer, I let the local milk age for several days and now I seem to be getting better results (although I am now at the cusp of its sell-by date). Perhaps there is some psychological effect going on and I am paying more attention to my technique now, though. If in fact this success is due to the aging, can anyone explain why? What happens over time that allows the milk to froth better?