Here are a bunch of mistakes I have made that resulted in STM32 not sleeping or not waking.

• IWDG not stopping
  • Conditions:
    • Option bytes → IWDG_SW bit: cleared (0)
    • MCU core is in Stop mode
  • Root cause:
    • Independent watchdog counter is running in Stop mode
  • Solution:
    • Option bytes → IWDG_STOP bit: cleared (0: Independent watchdog counter is frozen in Stop mode)
    • If STM32 part does not support IWDG_STOP, then you must wake the MCU before IWDG expires, reload IWDG then go back to Stop mode

• RTC not waking STM32 due to alarm not programmed
  • Root cause(s) could be any of:
    • Backup domain is protected
      • Solutions:
        • If keeping backup domain locked: Call HAL_PWR_EnableBkUpAccess() to set the DBP bit in the Power control register 1 (PWR_CR1) to enable access to the backup domain before setting RTC time, alarm or backup registers
        • If keeping backup domain unlocked: Ensure HAL_PWR_DisableBkUpAccess() is not called to prevent clearing the DBP bit inadvertently
    • LSE oscillation failed during shutdown
      • Solutions:
        • Verify __HAL_RCC_LSEDRIVE_CONFIG(__RCC_LSEDRIVE__) is called before setting RTC clock source to LSE, to set an appropriate drive level (refer to AN2867 link below)
          • Refer to STM32 part-specific HAL user manual for valid values for ''__RCC_LSEDRIVE__“'
          • Refer to AN2867 Guidelines for oscillator design on STM8AF/AL/S and STM32 MCUs/MPUs for drive level calculation
        • Verify LSE circuit against AN2867
        • Verify hardware pins for damage or contamination