Dunno, hon. Could be several things. Not enough ice? Not enought salt? Using a milk/cream (half n' half)mixture instead of only cream? Not shaking it enough or for long enough?

I had a pint of cream to use up last night and made a cup for DH and one for me and it worked fine. Used two sandwich-szie ziplocs for the cream mixture and a gallon size for the ice and salt. Mixed up both at once and shook them a bit longer.

Better luck with it next time...