Booting issue in mbed os with stm32f746zg

I made prototype board with stm32f746zg and mbed os.

I made a total of 10ea. but board didn’t boot board of 9ea. only one thing is booted.

I soldered the crystal(LSE, 32768hz) of the board that did not boot again. crystal. and then it was booted.
but… a few hours later, it didn’t boot again…

i don’t know why between booting and LSE crystal…

I knew there was no relationship between LSE and boot.

I’ll be waiting for your reply.