My 2 cenrts worth = I think you will find it is a combination of the material the star pickets are made of and the type of welding rods you are using. I had the same issue.
I am trying to remember when welding disimilair metals use 308 or 309 electrodes. Also sometimes you may actually need to change the polarity ie earth positive electrode negative, move the earth to a different location on the job to stop arc wander. Consider the angle of the electrode also as previous mentioned.
Finally - nothing worthwhile comes easy