From my experience, SPS are very sensitive to phosphate. Check your phosphate reading real quick and see what it is. Also, your Alkalinity has to be pretty constant and high. (~12dKh) Check your magnesium levels as well. There is a way you can keep SPS corals - but like they said above, you have to have a pretty well established tank.
When I kept SPS for a time, I regularly dosed kalkwasser to keep my calcium and alkalinity high six days a week and on the seventh day I wouldn't dose kalkwasser. I would dose a marine buffer to keep the pH/alk from dropping.
I find that it all depends on the maturity of the tank, the internal flow rate, the type of lighting and the chemical balance that makes SPS corals so difficult to keep.