Number of reasons for being slightly left, could be your sights need slight adjustment or could be your stance, grip, trigger pull etc.
I've found these training targets awfully helpful to correct any "faults" with my technique ....
http://www.outdoorchannel.com/News/Shoo ... _2008.aspx" onclick="window.open(this.href);return false;
Also at the bottom of that page, the deck of cards target is a fun game to play with friends at the range. See who can get the best 5-card poker hand etc.