Problems powering standalone arduino with USB powerbank
According to the datasheet, the LM317 needs an input voltage 3v higher than the output. Since 6.3&lt;5, there's not enough margin for the LM317.

You can use an AMS117 LDO @3.3, which only needs an input of 3.9v to operate. I find these modules very handy. If the MCU has some tolerances, you can likely get away with using a diode or two in series with the 5v rail to drop it down; the actual forward voltage fluctuates slightly on load, but it will likely stay in the acceptable range for your MCU.

you can also use a switch-mode regulator, commonly sold as "buck converters" on ebay, to more efficiently reduce the voltage. This is the way to go over linear regulator like the 317 if you power your project from batteries or have concerns about heat. I go through a lot of these style converters, and they seem to work very well.