"Your dog does not learn from you."
Translation:הכלב שלך לא לומד ממך.
How is הכלב שלך לא לומד מכם proper? It is saying your (single) dog doesn't learn from you (plural). But you can't really address two groups at once, can you?
It's possible if you are addressing only one person, but מכם references a group that the person is a part of.
Steve: Jim and I are trying to teach my dog to fetch.
You: Your [Steve's] dog is not learning from [the two of] you.
I agree with PennyFore - there should be agreement among the subjects. While it's possible jg9000's explanation is correct, it doesn't seem plausible in a teaching exercise.