Many people do not run the calcium hardness test to endpoint but stop at the color change, it is important to keep titranting until endpoint is reached. I have seen samples where the calcium hardness was over 600 ppm, which is over 60 drops of titrant with a 25 ml sample and many people thing that it cannot require that many drops! However, it can! In some parts of the country CH well over that reading is not uncommon, nor is it uncommon to have very high calcium with new plaster! My first suggestion is to do the 10 ml test instead of the 25 ml test (10 drops buffer, 3 drops of indicator, and each drop of titrant equals 25 ppm calcium hardness).
Second, SWIRL AFTER EACH DROP FOR 20 TO 30 SECONDS OR USE A MAGNETIC STIRRER! The reaction IS slow and can appear to 'stick' at the violet in between color for several drops, particularly with the higher resolution 25 ml sample test!
Third, the first reagent is sodium hydroxide which causing the magnesium in the sample to precipitate out so yo are only testing for calcium hardness and not total hardness and is a reason to really swirl well for about 30 seconds after adding the first reagent to make sure the reaction carries to completion! Once again, a magnetic stirrer is useful here!