I am learning to rig in Maya [2010] (for game design). I created a skeleton, smooth binded it to the model, and now am painting skin weights.

The problem: Every time i paint a skin weight, and then go to another weight and paint on it, when i go back to the 1st weight, it has dramatically changed! its completely different! so basically all progress is removed when go on to the next joint.

heres some screenshots showing what i mean: