For coffee makers, why won't my milk foam?

I work at a coffee shop and today our batch of milk wasn’t foaming properly for our preparation of cappuccino. We asked our manager what was causing this and she had been told that it is due to the diet of the cows that had produced the milk, as if the grass that they had been eating had changed due to the seasons and this had affected the milk.

I laughed at the idea, but does anyone here know if this is true or not?

http://www.jimseven.com/2006/12/16/why-wont-my-milk-foam/

I googled the issue and found this article which taught me about glycerol, but didn’t mention anything about a cow’s diet being a potential problem.