Do you see a tiny dropdown button next to 'Reply'? If you click it, it will show you three options at the bottom of the fold-out menu, saying 'Button does...'. You can set that to Reply-all, if you want.

I use Pegasus 4.51 (June 23, 2009) under Windows XP.  Everything was working fine until recently when the "Reply to all" feature stopped working.  It responds only to the person from whom the message came, but not the others.