Your mallet needs to have a good bit of mass to be effective.
Here is a link to a thread on this site from a while back:
http://allmetalshaping.com/showthrea...hlight=mallets
You may be interested in the TuckPuck® for shrinking and stretching. You can go to YouTube and view the 4 part instructional video which may give you some new information. I need to put up a current video because I have found that solid mounting of the TuckPuck® is necessary to be effective and that a heavy mallet is also needed. The website is in my signature below.
http://www.youtube.com/watch?v=KEgnRNcn7BQ
http://www.youtube.com/watch?v=oShUN...eature=related
http://www.youtube.com/watch?v=KRmY9...eature=related
http://www.youtube.com/watch?v=vw7--...eature=related