There is not a lot of info we could go on about what might cause this.
Apart from the known but not yet solved issue with “brown-out” conditions and some occasional disconnets due to SIM contact issues on ~1% of devices (blinking blue) the stability is rather good in my experience.
Just a hint
particle flash --usb firmware.bin
is much more reliable when it comes to actually telling you whether a flash succeeded or not.
But as said, we need more info.
How do you exactly power the devices?
Even if it’s simple, how exactly does your code look?
Have you considered SYSTEM_THREAD(ENABLED) and/or a non-AUTOMATIC SYSTEM_MODE()?