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poco m3 not turning on

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ok guys i know there are loads of threads about thois but ive got a slightly differen situation.
my daughter had this phone and then it suddenly tured off and wouldn't turn back on, i managed to get it to boot by disconnecting the battery and plugging it while holding the power down and reconnecting the battery which put me into fastboot then the boot to edl script made the phone boot. i have since unlocked the bootloader and flashed twrp and the reflashed the stock firmware which worked for a few hours but i tried to reboot it and now it wont boot, i can get to twrp but even if i flash the stock firmware it refuses to boot any idea on what i should do to try to get this phone to boot up?
Try to flash a custom rom, as far as i know this problem is caused by a MIUI update.
i have tried to flash a couple but every time i try it fails with error 7 and tried the normal fix but it doesnt help. then anything i do in twrp makes the system unmount and then it cant mount again. the only eay to get to get it to boot again is to take the back off and disconnect the battery and hold the power button down when i reconnect it but ive got no idea why it does this.