How's your magnesium? If low, it will inhibit the ability of calcium utilization.
Articles of interest:
Solving Calcium and Alkalinity Problems:
http://www.advancedaquarist.com/issues/nov2002/chem.htm
Magnesium in Reef Aquaria:
http://www.advancedaquarist.com/issues/oct2003/chem.htm
The Reef Chemistry Calculator:
http://home.comcast.net/~jdieck1/chem_calc3.html
Aquarium System Volume Calculator:
http://home.comcast.net/~jdieck1/volcalc.html