I had a similar problem with a PIC18F46J11 which lost several seconds every minute. The crystal WAS running at 32768Hz exactly as intended. I don't know if the same applies to your PIC24 device because you haven't told us which one it is, check you are using the crystal oscillator as your timing source. Everything seemed to work properly, including the alarm registers but just a little too slow. The problem was one bit in one register which was making the RTC module derive it's source from the internal slow clock. Unfortunately, it runs at 31KHz not 32.768KHz !! Being so close in frequency it's easy to be misled about the problem.
Brian.