Why don't I get the same amount of contraction on various muscle groups at a given mA setting?

Not only will you not get the same degree of contraction between different muscle groups at a given mA setting, but the feel of the contraction from the various muscle groups will be different. This is due to a number of factors such as percent composition of slow and fast twitch muscle fibers and size of the muscle. This is normal. Expect the mA settings used while training various muscle groups to vary. Let trial and error, feel of the stimulation, and degree of contraction guide you.