hi all, when rtc overflow? this will happen in my use env. does the rtc counter auto reset to zero or stop count?

Hi, all , please help me . I use stm32L151. when rtc wake up timer is 30 or smaller minuts, the board can wake up, and work . but when rtc timer is 6 or long hours, the board some times not wake up . what happened ? I need a long sleep (many hours)...