Is this a new install? If not, perhaps the cable slipped where it attaches to the der.
You should only have a small amount of slack on the cable when on the small ring. If you can't take the slack out using the barrel adjuster on the shifter, you will have to loosen the cable from the der. Then reattach it so that there is less slack in the cable. Reset the barrel adjuster and put the shifter in low before doing this.
Bookmark this site. Lots of info.
http://www.parktool.com/blog/repair-help/front-derailleur-adjustments
You should only have a small amount of slack on the cable when on the small ring. If you can't take the slack out using the barrel adjuster on the shifter, you will have to loosen the cable from the der. Then reattach it so that there is less slack in the cable. Reset the barrel adjuster and put the shifter in low before doing this.
Bookmark this site. Lots of info.
http://www.parktool.com/blog/repair-help/front-derailleur-adjustments