FOTA over wifi

I try to download FOTA from http server to nrf7002 over WIFI

i use fota download library and initialized it ,then call API fota_download_start()
then the fota_callback_handler respond with   "FOTA_DOWNLOAD_EVT_FINISHED" , but when i call  sys_reboot(SYS_REBOOT_COLD) the system doesn't jump to the new firmware and reset to the old firmware

so where's the problem ?