The main reason yeast doesn't froth in the milk is because the milk temperature is not right. This is why food thermometers were made, lol. The milk needs to be 100°F-110°F like it says in the back of the yeast packet. If you are like me and don't own a thermometer, you will need to gauge the temp by your hands. The milk should be lukewarm like you are going to feed it to a baby.