Sounds like your milk is sitting at sub freezing temperature at some point in its life. If the water content in milk is allowed to begin freezing (even if the whole carton/jug isn't frozen), you won't be able to create awesome microfoam - it has to do with the interaction of frozen water and fat globules, if I understand correctly.
Best check all of your fridges, make sure that they are between 34-44 (or whatever your shop standard is). If your fridges are all to spec, and your milk is still behaving like this, have a chat with your dairy vendor. I've seen it be as simple as milk being transported right next to the cooling agent in a refrigerated truck.