Cables are funny sometimes. As others have said make sure you have routed the cable properly. It helps sometimes to take the cable out and reinstall it as the cable may be in a bind even if it's in the the correct location. You may have also purchased a bad cable, rare but possible.
Take the cable you just removed, (the new one) and pull it back and forth inside the sleeve and see if it is still hard to slide.
To adjust the cable with a stock quadrant, take your foot and place it underneath the clutch pedal, and pull up. You will hear a few clicks. You have now adjusted the cable. Also I have heard of people depressing the clutch pedal and sidestepping the clutch. This will probably net the same result when the pedal is on its way back up at such speed and force.
If a Cable gets hot it can make it hard to push the clutch in as well.
Cables are funny sometimes, it just takes some fooling with and you may need to reinstall it a time or two if it isn't right.
Hope this helps, Good Luck.