zgemma s2 reflashes but always back to boot

hi all looking for help..
my s2 keeps going to boot mode..
it reflashes with any image and can set it up etc and all working but as soon as box restarts or gets switched off it'll not go off the boot mode again ?? tried different usb tried different images ??
if I kept it switched on all the time it's fine in till it goes off ??
any help be great thanks

when use boot loader comes up flash then usb then flash usb boot flash usn boot?