After an alarm event occurs, you will need to read the interrupt status register in order to clear it but you will also need to clear the control register (0xFFh) bit Alarm Flag Arm. This bit must be written to 0 to rearm the alarm and it goes to 1 at the same time as the AF bit in the interrupt register. Once the alarm has been fully reset the alarm registers RTC_match and RTC_ctrl can be changed.