If it sometimes works and sometimes doesn’t, that sounds vaguely similar to this other issue that was recently reported:
The conclusion there seems to be that the problem might be related to some sort of power management work in recent kernels. Can you get one of the drives to work consistently either by increasing the available power (e.g. running the system plugged-in instead of on the battery) or by reducing the power that is being drawn from associated USB ports (e.g. disconnect all your other USB devices or plug them into a different jack panel).