use line numbers anyway instead of g...
ex ,. try
1,$s/a/b/
if not
use
1,(n)s/a/b/
that should work.. then you need to figure out why the g flag is not working.. it may be that you are not using sed according to the file syntax... it could be you need to do something like this s/\<a\>/b/ i can't see the file so I can't tell you.. try this site for help
http://sed.sourceforge.net
moxxx68