I live in Burnie, and I find that Betta full cream milk is usually good. Occasionally get some that doesnt work as well as normal but not consistently over the time frame you mentioned. I also find Lite milk doesnt ever seem to produce same quality of microfoam as is possible with the full cream. I have no idea about the supermarket brands, will not buy that.
I have recently tried some UHT full cream milk (nothing else in the house), and it was surprisingly good for a latte style drink. Have you tried adding some UHT to your bad batches of full cream?